refs #353537326: Use fully qualified class name for BaseTestRunner in takeScreenshotIfTestFailed()

The method takeScreenshotIfTestFailed() attempts to check if PHPUnit <10 is in use by calling class_exists(BaseTestRunner::class), but this always returns false unless a use statement is present.

Using the fully qualified class name \PHPUnit\Runner\BaseTestRunner::class ensures the check works without requiring a use statement, preserving compatibility with both PHPUnit 9 and PHPUnit 10.

Closes #3537326