Add $wp_filters to WP_UnitTestCase_Base::_backup_hooks() and ::_restore_hooks()

[SergeyBiryukov]

Background: #28535, #35357.

• [29251] / #28535 introduced saving the state of filter-related globals ($wp_actions, $wp_current_filter, and $wp_filter) between running individual unit tests, so that all tests initially use the same filters/actions.
• [53803] / #35357 added a new global, $wp_filters. It should be reset too, along with $wp_actions.

The change should be made in WP_UnitTestCase_Base::_backup_hooks()​ and ::_restore_hooks()​.

Backs up $wp_filters global before running test cases

Trac ticket: https://core.trac.wordpress.org/ticket/57236

[SergeyBiryukov]

In 55160:

Build/Test Tools: Back up and restore the $wp_filters global when running tests.

Introduced along with the did_filter() function, the $wp_filters global stores the number of times each filter has been applied during the current request.

This commit adds $wp_filters to the list of globals that are saved in WP_UnitTestCase_Base::_backup_hooks() and restored in ::_restore_hooks() so that hooks set by the current test do not accidentally affect future tests.

Additionally, this commit brings some consistency by backing up and restoring the hook-related globals in the same order they are defined in wp-includes/plugin.php.

Follow-up to [29251], [53803].

Props sanjucta, SergeyBiryukov.
Fixes #57236.

Thanks for the PR! Merged in r55160.