[Testing] Use fresh single-file source locator under PHPUnit to avoid stale reflection

[TomasVotruba]

Problem

Rule tests write each fixture to a throwaway temp file and delete it in tearDown(). The shared OptimizedSingleFileSourceLocatorRepository caches single-file locators by path for the whole worker and never evicts them. So a stale locator for a since-deleted fixture file can leak into a later test that runs in the same parallel worker.

Because fastunit groups tests into worker chunks by weight, whether this bites depends on how the chunks fall out - adding or removing test classes reshuffles the grouping. That makes it an intermittent, order-dependent failure: a test can go red not from its own change but from what it happens to share a worker with.

It reproduces reliably at a low worker count (matching CI's core count):

vendor/bin/fastunit -p 4 tests rules-tests utils/phpstan/tests

The symptom is a rule that resolves reflection (e.g. a parent property) suddenly seeing incomplete data - the parent property becomes invisible and the expected change is not applied - accompanied by hash_file(... /Fixture/fixture.php): Failed to open stream: No such file warnings from the stale locator.

Fix

Under PHPUnit, build a fresh single-file locator via OptimizedSingleFileSourceLocatorFactory::create() instead of the cached getOrCreate(). Production keeps the cached repository, where files are stable and caching is a performance win. The isPHPUnitRun guard already exists in provide() for the aggregate locator; this extends the same reasoning to the single-file locators.

Validation

Full suite green across worker counts that previously failed:

• vendor/bin/fastunit -p 4 … -> OK
• vendor/bin/fastunit -p 3 … -> OK

composer check-cs, composer phpstan, and composer rector pass on the changed file.