The serialization test cases look for files based on the name of the class they are testing. After the namespacing of ParserOutput, they are looking for files named like:
1.42-MediaWiki\Parser\ParserOutput-binaryPageProperties.json
The embedded backslashes in these filenames would raise havoc on Windows machines. But what's more, none of the existing ParserOutput tests will actually be checked anymore because the filenames don't match up with what is expected after namespacing.
Change #1023578 had a related patch set uploaded (by C. Scott Ananian; author: C. Scott Ananian):
[mediawiki/core@master] Serialization test cases: fix filename after ParserOutput namespacing
Change #1023578 merged by jenkins-bot:
[mediawiki/core@master] Serialization test cases: fix filename after ParserOutput namespacing
Change #1032821 had a related patch set uploaded (by Reedy; author: C. Scott Ananian):
[mediawiki/core@REL1_42] Serialization test cases: fix filename after ParserOutput namespacing
Change #1032821 merged by jenkins-bot:
[mediawiki/core@REL1_42] Serialization test cases: fix filename after ParserOutput namespacing