Add regression test for wp_option theme data corruption bug

[himanshupathak95]

Trac ticket: 53520

This PR adds a regression test for the bug fixed in Gutenberg v10.7.4 where updating options in options-general.php would corrupt theme-related wp_options.

The bug caused themes to break and revert to default themes after updating any setting in the Settings or Customizer pages. This was tracked in wp-calypso issues #53447 and #53431 and fixed in Gutenberg PR #32797.

This test ensures that updating an unrelated option doesn't corrupt theme-related options like 'stylesheet' and 'template', preventing this serious issue from recurring in the future.

See: WordPress/gutenberg#32797

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