Remove almost duplicate tests

We have some tests that are almost copy-pasted for testing mx structure as well. With mx we have some more commands as we connect to the workers but most of the queries are the same. It is hard to maintain two test files. With some conditions (such as is_mx) in the test file, we can have a single file, of course with an alternative output. However this would have the advantage that whenever we update the single file, we would also see the effect on the mx side (as the alternative output will change). With the current structure it is possible to forget to update one of the test files.

See: https://github.com/citusdata/citus/pull/4133#discussion_r484422432