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.