make -C src/test/isolation failure in index-killtuples due to btree_gist

Hi all,
(CCing committer)

The following command fails, because btree_gist is not installed in
the context of the isolation tests:
make -C src/test/isolation/

This test has been added recently by 377b7ab14524.  Some efforts have
been done lately to remove any dependency to contrib/ in
src/test/regress/, like b1720fe63f34, and it does not strike me as a
good idea to begin doing that in the main isolation test suite, so
perhaps the best thing to do here is just move this test to
contrib/btree_gist/?  Rewriting this test without the dependency to
the contrib module seems tricky, and there's the matter of the test
correctness.

Regards,
--
Michael