Add some initial PHPUnit tests for `WP_Filesystem_Direct`

[costdev]

This is a tests-only ticket.

WP_Filesystem_Direct is, by far, the most used filesystem abstraction class.

While some PHPUnit tests exist for a WP_Filesystem_MockFS class, this ticket aims to add PHPUnit tests for the WP_Filesystem_Direct class itself.

The other filesystem abstraction classes, WP_Filesystem_FTPext, WP_Filesystem_ftpsockets and WP_Filesystem_SSH2, do not have PHPUnit test runner configurations. While in future, we may reuse many of the same tests across filesystem abstraction classes, this ticket specifically targets WP_Filesystem_Direct.

This adds some initial tests for the WP_Filesystem_Direct filesystem abstraction class.

These tests do not aim to cover all paths, but to add line/branch coverage where source changes are not needed to facilitate testing or resolve bugs. The intention is to make it easier to know how to hit a line/branch when trying to cover paths in future.

This also introduces the WP_Filesystem_Direct_UnitTestCase class, which contains shared properties, set_up()/tear_down(), helpers and common data providers for the tests.

To allow constants to be set without affecting other tests in the test suite, some of these tests use the @runInSeparateProcess annotation ​[Ref]. To prevent an exception for Serialization of 'Closure' is not allowed, the @preserveGlobalState disabled annotation is also used ​[Ref].

Tests exist for the following methods:

• __construct()
• get_contents()
• get_contents_array()
• put_contents()
• cwd()
• chdir()
• chgrp()
• chmod()
• chown()
• getchmod()
• copy()
• move()
• delete()
• exists()
• is_file()
• is_dir()
• is_readable()
• is_writable()
• atime()
• mtime()
• size()
• touch()
• mkdir()
• rmdir()
• dirlist()

Trac ticket: https://core.trac.wordpress.org/ticket/57774

[costdev]

​PR 4106 is ready for review.

Line coverage: 74.56%
Branch coverage: 76.83%

To cover more lines/branches, source changes are needed to facilitate testing/fix bugs. This will be done in separate tickets.

[hellofromTonya]

Self-assigning for review and commit consideration.

[costdev]

This ticket was discussed in the bug scrub.

While this is a tests-only ticket and can be committed at any time, we're approaching RC1 and the PR is awaiting review, so I'm going to move this ticket to the 6.3 milestone.

Should it be reviewed and ready for committed prior to 6.2 RC1, feel free to bring it back into the 6.2 milestone for commit.

Additional props: @mukesh27

[oglekler]

@hellofromTonya, please take a look at this one 🙏

[hellofromTonya]

Moving into the 6.4 milestone.

@costdev sorry for the big delay. I'll get your PR reviewed and then committed.

[oglekler]

The patch needs to be refreshed, most likely the part in phpcs.xml.dist is not needed anymore, see:
​https://github.com/WordPress/wordpress-develop/commit/60d2d8ac75c4350a45e4c6970c7d0295b757f1f0

Great job, btw. Let's try to make it into the current milestone :)

[costdev]

Thanks @oglekler!

​PR 4106 has now been rebased on trunk and the conflict with phpcs.xml.dist resolved.

[oglekler]

This ticket was discussed during bug scrub.

@hellofromTonya can you please review this PR?

Add props to @mukesh27

[hellofromTonya]

As this ticket is for adding automated tests, it is not bound to the 6.4 release milestones and can be committed at any time before the 6.4 Dry Cycle starts.

[hellofromTonya]

To avoid confusion for 6.4 triagers during the beta cycle, I changed the component to Build/Test as this ticket tracks only tests.