Tests: Add unit tests for wp_page_reload_on_back_button_js()

[pbearne]

This ticket adds unit tests for the wp_page_reload_on_back_button_js() function in wp-admin/includes/misc.php. The tests will verify the correct HTML/JavaScript output.

Description:
This PR adds unit tests for the wp_page_reload_on_back_button_js() function in wp-admin/includes/misc.php. This function outputs a small JavaScript snippet designed to force a page reload if the user navigates to the page using the browser's back or forward buttons, ensuring that content (like post titles and editor state) is not stale.

The tests cover:

• Verification of the <script> tag output.
• Verification of the performance.navigation.type === 2 check (which detects back/forward navigation).
• Verification of the document.location.reload( true ) call.

Trac ticket: https://core.trac.wordpress.org/ticket/65193

AI Disclosure:

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

[desrosj]

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

[SergeyBiryukov]

In 62367:

Tests: Add unit tests for wp_page_reload_on_back_button_js().

Follow-up to [37619].

Props pbearne.
Fixes #65193.