At the time tests were backported from Gutenberg, a block template theme was not available in the test suite; thus, the test was marked as `markTestIncomplete()`.
Now that `block-theme` and `block-theme-child` test fixture block template themes are available, this commit removes the `markTestIncomplete()` and adjusts the test to run by switching to the `block-theme` test fixture.
Follow-up to [51003], [52062], [52247].
Props bernhard-reiter.
Fixes#54551.
git-svn-id: https://develop.svn.wordpress.org/trunk@52314 602fd350-edb4-49c9-b593-d223f7449a82