Tests: Add unit tests for wp_reset_vars()

[pbearne]

This ticket adds unit tests for the wp_reset_vars() function in wp-admin/includes/misc.php. This function resets global variables based on $_GET and $_POST values.

Description:
This PR adds unit tests for the wp_reset_vars() function in wp-admin/includes/misc.php. These tests ensure that the function correctly resets global variables based on the values in $_POST and $_GET, or defaults them to an empty string.

The tests cover:

• Resetting variables from $_POST when they are not empty.
• Resetting variables from $_GET when they are empty in $_POST (including cases like '0' or '').
• Defaulting variables to an empty string when they are missing from both $_POST and $_GET.
• Handling multiple variables simultaneously.

Trac ticket: https://core.trac.wordpress.org/ticket/65180

AI Disclosure:

• AI assistance: Yes
• Tool(s): Junie (JetBrains)
• Model(s): gemini-3-flash-preview
• Used for: Code analysis, test implementation, and workflow management.

[darshitrajyaguru97]

Tested the patch locally and confirmed the new wp_reset_vars() tests are passing successfully.

Environment:

• WordPress: trunk
• PHP: 8.2.29
• Database: MySQL
• PHPUnit: latest Core test suite

The added coverage looks solid overall, especially around documenting the current empty() behavior for values like '0' and ''.

A few additional edge cases may be worth considering, either in this patch or a potential follow-up:

• false
• 0 (integer)
• null
• array values

Additionally, in tear_down(), previously unset globals could potentially be restored using unset() instead of assigning null, which may help preserve the original global state more accurately.

Patch tests successfully.

[desrosj]

Adding the tests focus, which is used to indicate a ticket is solely focused on adding tests.