PHPUnit tests: Add/improve @covers annotations for WP 6.3

[pbearne]

Previously:

• #56782 (6.2)
• #39265 (up to 6.1)

Continuation of ongoing work to add and/or improve the PHPUnit tests @covers annotations for more accurate coverage reporting.

[hellofromTonya]

In 55565:

Tests: Add test class for wp_script_is().

Changes:

• Adds a test class for wp_script_is().
• Moves WP_Dependencies_jQuery::test_wp_script_is_dep_enqueued() into this test class and splits it into 2 separate tests, happy and unhappy paths.
• Adds WP_Scripts::query @covers, as wp_script_is() is helper function for it.
• Relocates the global resetting from the test method to the clean_up_global_scope() method.

Follow-up to [52010], [29252].

Props hellofromTonya, antonvlasenko.
See #57841, #57958.

[hellofromTonya]

In 55572:

Tests: Rename test class and improve tests for wp_get_global_stylesheet().

Changes include:

• Renames the test class to be compliant with test coding standards.
• Converts the test class to extend WP_Theme_UnitTestCase to reuse test fixtures.
• Implements data providers to encapsulate datasets and reduce code repetition.
• Adds a @covers annotation.
• Improves assertion messages to help with diagnosing failed tests.

Follow-up to [55567], [55148], [52682], [53916], [52675-52677].

Props costdev, hellofromTonya.
See #57841, #57958.

[hellofromTonya]

With 6.3 now in RC cycle, closing this ticket.

#58851 is open for the 6.4 cycle.