CI: Yet again add more locking to prevent parallel tests from colliding

[tbradsha]

Closes MONOREP-222

As done in #45851 and #45902, this adds locking to prevent parallel tests from colliding.

It seems Sync is the main culprit; perhaps if this continues to be an issue we should add locking to the Jetpack_Sync_TestBase class.

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Code Coverage Summary

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Full summary · PHP report · JS report

[anomiex]

I'm not seeing any theme stuff in this file. What is colliding here?

[tbradsha]

The issue is triggered here:
https://github.com/Automattic/jetpack/actions/runs/19333566055/job/55302450703#step:11:11641

My guess is that $this->server_event_storage->reset(); is wiping things out during the Sync Users test, and that interferes with Theme tests.

[anomiex]

Looking at the error message there, it seems more likely that it's a race where the Users test somewhere inside of sync calls search_theme_directories() (probably via wp_get_themes()) and gets past line 515, then the other process cleans up the directory before it reaches line 539 for that theme.

If that's what's going on here, then it could probably happen in any of these tests and the idea of locking in Jetpack_Sync_TestBase may be the best one. But we may want to see if Vulcan has any better insight here.

@Automattic/jetpack-vulcan: TL;DR: When running coverage tests, we run the normal and multisite tests in parallel to save some wall clock time. Sometimes this makes Sync tests fail because both are using the same themes directory in the filesystem. We already added locking to Jetpack_Sync_Themes_Test (the one that actually writes the temporary themes) in #45851, and Jetpack_Sync_Plugins_Test (which writes a the/the.php plugin) in #45902. But it's looking like we might need to just lock all the Sync tests, since even filesystem-read-only ones can get broken if one of the filesystem-writing tests writes the wrong thing in the other process. Does that seem right to you?

[anomiex]

If we do wind up deciding that "lock everything" is the way to go, probably we should mostly revert #45851 and #45902 in favor of a trait that provides the locking functions.