Found it! The JsonConfig extension has LUA integration tests which require Scribunto. To avoid failing when Scribunto is not loaded, the php class has an early return:

<?php

namespace JsonConfig\Tests; 

use MediaWiki\Extension\Scribunto\Tests\Engines\LuaCommon\LuaEngineTestBase;

if ( !class_exists( LuaEngineTestBase::class ) ) {
    return;
}

class JCLuaLibraryTest extends LuaEngineTestBase {
...
    public static function suite( $className ) {
        self::doMock();
        try {
            return parent::suite( $className );
        } finally {
            self::doUnmock();
        }
    }

I do not understand how:

• the JCLuaLibraryTest gets loaded given the class does not exist and the php file containing the class has a return
• the traces refers to MediaWikiIntegrationTestCase::suite but the class extends non existing LuaEngineTestBase.

Maybe it is an oddity of the test splitter.

I gave it a try a few times and could not reproduce since the splitter could not find GlobalJsonLinksTest and expected .../GlobalJsonLinksTest.php. I suspected

• something wrong in the autoloader
• PHPUnit files discovery misbehaving somehow
• the splitter mishandling the data structure somehow

I have the MediaWiki repositories cloning using the same tree as in Gerrit (ex: ~/projects/mediawiki/core and ~/projects/mediawiki/extensions/JsonConfig) with $wgExtensionDirectory = "../extensions";.

Turns out it uses RecursiveDirectoryIterator which does not follow symbolic links by default and I had JsonConfig symlinked from ../../extensions). I fixed it by cp -r inside core extensions directory, updated $wgExtensionDirectory et voilà! Fixed.

I ran PHPUnit and reproduced the issue.

Once I have managed to reproduce the issue, I went to try to reduce the scope in order to find a quick/easy reproducible. The issue occurs when using solely FlaggedRevs :

wfLoadExtension( 'FlaggedRevs' );
wfLoadExtension( 'JsonConfig' );

The reason is, and excuse my swearing, A CLASS ALIAS FOR A CLASS FROM ANOTHER EXTENSION:

<?php

use MediaWiki\Extension\Scribunto\Tests\Engines\LuaCommon\LuaEngineTestBase;
use MediaWiki\Title\Title;    

if ( !class_exists( LuaEngineTestBase::class ) ) {
    class_alias( MediaWikiIntegrationTestCase::class, LuaEngineTestBase::class );
}

That was introduced by fb4eaba9f9ab199bd91ec918a4298500188a6582 / https://gerrit.wikimedia.org/r/c/mediawiki/extensions/FlaggedRevs/+/774435:

Stub base class so that test doesn't break when Scribunto is missing
    
When the Scribunto extension is absent, a base test class cannot be
loaded, which is fatal to the PHP parser.  Work around by providing a
class alias.

JsonConfig has the same requirement (skipping tests when Scribunto is not loaded) but solves it with an early return:

<?php

namespace JsonConfig\Tests;

use MediaWiki\Extension\Scribunto\Tests\Engines\LuaCommon\LuaEngineTestBase;

if ( !class_exists( LuaEngineTestBase::class ) ) {
    return;
}

class JCLuaLibraryTest extends LuaEngineTestBase {

And since the class alias fulfills class_exists() expectations, the JCLuaLibraryTest get defined, PHPUnit invokes its suite() and that explodes.

Why do I get FlaggedRevs? Because Kartographer depends on both JsonConfig and FlaggedRevs

I think the easiest is to make FlaggedRevs to do an early return, the same it is done by JsonConfig

🎉 🎂 🕯

ProofreadPage does the same:

if ( !class_exists( LuaEngineTestBase::class ) ) {
	return;
}

Change #1217777 had a related patch set uploaded (by Hashar; author: Hashar):

[mediawiki/extensions/FlaggedRevs@master] Skip test class when LuaEngineTestBase is not available

https://gerrit.wikimedia.org/r/1217777

Change #1217778 had a related patch set uploaded (by Hashar; author: Hashar):

[mediawiki/extensions/FlaggedRevs@REL1_45] Skip test class when LuaEngineTestBase is not available

https://gerrit.wikimedia.org/r/1217778

Change #1217779 had a related patch set uploaded (by Hashar; author: Hashar):

[mediawiki/extensions/FlaggedRevs@REL1_44] Skip test class when LuaEngineTestBase is not available

https://gerrit.wikimedia.org/r/1217779

Change #1217780 had a related patch set uploaded (by Hashar; author: Hashar):

[mediawiki/extensions/FlaggedRevs@REL1_43] Skip test class when LuaEngineTestBase is not available

https://gerrit.wikimedia.org/r/1217780

Change #1217781 had a related patch set uploaded (by Hashar; author: Hashar):

[mediawiki/extensions/FlaggedRevs@REL1_39] Skip test class when LuaEngineTestBase is not available

https://gerrit.wikimedia.org/r/1217781

This will let us disable recursion on Kartographer (which depends on FlaggedRevs and will no more have Scribunto)

Change #1217784 had a related patch set uploaded (by Hashar; author: Hashar):

[integration/config@master] Zuul: [Kartographer] disable recursion

https://gerrit.wikimedia.org/r/1217784

Change #1217777 merged by jenkins-bot:

[mediawiki/extensions/FlaggedRevs@master] Skip test class when LuaEngineTestBase is not available

https://gerrit.wikimedia.org/r/1217777

Change #1217781 merged by jenkins-bot:

[mediawiki/extensions/FlaggedRevs@REL1_39] Skip test class when LuaEngineTestBase is not available

https://gerrit.wikimedia.org/r/1217781

Change #1217779 merged by jenkins-bot:

[mediawiki/extensions/FlaggedRevs@REL1_44] Skip test class when LuaEngineTestBase is not available

https://gerrit.wikimedia.org/r/1217779

Change #1217778 merged by jenkins-bot:

[mediawiki/extensions/FlaggedRevs@REL1_45] Skip test class when LuaEngineTestBase is not available

https://gerrit.wikimedia.org/r/1217778

Change #1217780 merged by jenkins-bot:

[mediawiki/extensions/FlaggedRevs@REL1_43] Skip test class when LuaEngineTestBase is not available

https://gerrit.wikimedia.org/r/1217780

Change #1217784 merged by jenkins-bot:

[integration/config@master] Zuul: [Kartographer] disable recursion

https://gerrit.wikimedia.org/r/1217784