Tests: Add unit tests for wp_heartbeat_set_suspension()

[pbearne]

This ticket adds unit tests for the wp_heartbeat_set_suspension() function in wp-admin/includes/misc.php. The tests will verify that the function correctly modifies the Heartbeat settings to disable suspension when on post.php or post-new.php, and leaves them unchanged otherwise.

…/misc.php
Description:
This PR adds unit tests for the wp_heartbeat_set_suspension() function in wp-admin/includes/misc.php. This function is responsible for disabling the suspension of Heartbeat on the Add/Edit Post screens to ensure consistent behavior during content creation.

The tests cover:

• Disabling suspension when the $pagenow global is post.php.
• Disabling suspension when the $pagenow global is post-new.php.
• Ensuring suspension settings remain unchanged on other screens (e.g., index.php, edit.php).

Trac ticket: https://core.trac.wordpress.org/ticket/65200

AI Disclosure:

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

[SergeyBiryukov]

In 62351:

Tests: Add unit tests for wp_heartbeat_set_suspension().

Follow-up to [26428], [26549].

Props pbearne.
Fixes #65200.