Link color support (experimental): Add E2E Test

[ockham]

10.3.0 had a regression (#29535) where for traditional/PHP based themes with add_theme_support( 'experimental-link-color' );, the link color option was available, but the selected color was not applied in the editor or the frontend. Affected themes included Twenty Twenty-One (TT1) -- whereas block-based (FSE) themes (such as TT1 Blocks) were unaffected. The regression was fixed by #30452.

This comment provides good instructions for how to reproduce the issue, and the fix.

We should add an E2E test (based on that comment) -- probably in packages/e2e-tests/specs/experiments -- to guard against regressions like that.

cc/ @WunderBart @fullofcaffeine @rafaelgalani -- any takers? 😄