Tests: Remove `external-http` group from a `get_theme_feature_list()` test.

This particular test checks the list of theme features hardcoded into Core and does not perform an external API request.

Follow-up to [39906].

See #64225.

git-svn-id: https://develop.svn.wordpress.org/trunk@62243 602fd350-edb4-49c9-b593-d223f7449a82