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