WikibaseLexeme uses PHP implementation of Eris library for property-based testing.
The library is not part of mediawiki/core's dev dependencies. It is only defined in the extension's composer.json.
On Jenkins CI, we run PHPUnit test using the composer-flavoured version of the job, i.e. in my understanding composer install is being also run in the extension's directory (this seems to happen looking at job logs), and thanks to "load_composer_autoloader" set to true in extension.json, extension's autoload data is picked up by mediawiki.
Until Friday May 25th phpunit jobs seemed to be running just fine, see https://gerrit.wikimedia.org/r/#/c/435131/1, and https://integration.wikimedia.org/ci/job/mwext-testextension-hhvm-composer-jessie/14002/console.
For some reason since yesterday all phpunit job on this extension are failing due to autoloading issue, see e.g. the same patch https://integration.wikimedia.org/ci/job/mwext-testextension-hhvm-composer-jessie/14002/console, and: https://integration.wikimedia.org/ci/job/mwext-testextension-hhvm-composer-jessie/14073/console.
The issue does not seem to be hhvm-specific: https://integration.wikimedia.org/ci/job/mwext-testextension-php70-composer-jessie/684/console.
The main suspect is currently some recent changes in mediawiki's autoloading, but I didn't really find any significant changes made there over the weekend.
Attempt to explicitly defined the autoload namespace for the library didn't seem to help in any way, see: https://gerrit.wikimedia.org/r/#/c/435758/5/extension.json.
Any help would be appreciated. I assume adding the library to mediawiki/core's dev dependencies would help but that 1) seems a bit backwards, 2) does not explain why the autoloading issues suddenly popped up.