Add regression test for the wp_option data corruption bug

[fullofcaffeine]

Gutenberg v10.7.x introduced a bug that caused some wp_options to get blank or corrupted data after (any) settings were updated in the options-general.php page. This was eventually fixed in v10.7.4, with this PR: ​https://github.com/WordPress/gutenberg/releases/v10.7.4. See ​https://github.com/Automattic/wp-calypso/issues/53447 and ​https://github.com/Automattic/wp-calypso/issues/53431.

There's already a regression E2E test in Gutenberg that was introduced in this PR: ttps://github.com/WordPress/gutenberg/pull/32797. Ideally, though, it'd be either moved to WP core or also implemented here. It doesn't need to be an E2E test, and could be a lower-level integration test, if we can figure out how to reproduce at a lower level (there was a failed attempt to do this, though, see this comment: ​https://github.com/WordPress/gutenberg/pull/32797#issuecomment-865005516.

[hellofromTonya]

Today is 5.9 Feature Freeze. I'll punt this to Future Release (as 6.0 isn't
available yet for selection).

Test and build tool changes are allowed after feature freeze up until RC. If someone is able to work on this before then, it can always be moved back.

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: ​https://github.com/WordPress/gutenberg/pull/32797