[Filesystem] FilesystemTestCase is not compatible with PHPUnit 8

[GaryPEGEOT]

Symfony version(s) affected: >= 3.4

Description
the FilesystemTestCase has the same issue that #30071

How to reproduce
Run a test extending FilesystemTestCase with PHPUnit 8 will produce:
PHP Fatal error: Declaration of Symfony\Component\Filesystem\Tests\FilesystemTestCase::setUpBeforeClass() must be compatible with PHPUnit\Framework\TestCase::setUpBeforeClass(): void

Possible Solution
Use the same solution that #30474

[dmaicher]

Is this a test class that is meant to be used in userland code? I mean a lot of internal Symfony tests are not PHPUnit 8 compatible.

[GaryPEGEOT]

Not sure in fact... but it's not marked as internal

[xabbuh]

All classes from the Tests namespaces are purely internal code and you should never rely on any of them.

[chalasr]

Closing as explained.