Split Tests

It seems that `check-isolation` is one of the bottlenecks in our test suite in terms of the run time and it should be easy to split isolation tests: https://app.circleci.com/insights/github/citusdata/citus/workflows/build_and_test/jobs?branch=b1d8acd4-8701-4c76-8a87-7ff2320aa95f&reporting-window=last-30-days

A more Circleci way of splitting the tests would be the following:(which is much more work but more ideal and long term solution, for the whole test suite, not just check-isolation):
- We modify the tests so that they pass when run as a single test on top of check-base
- At this point, each test file can be run separately hence we use `circleci tests split` similar to https://github.com/citusdata/citus/blob/7d0f4f11c30a0de0f77041480a704185f179103a/.circleci/config.yml#L233.
- Circleci will then scatter the tests around containers and increasing the parallelism will be a single line change.

(Circleci can also scatter the tests by runtime, so it would be quite good in our case)

The above would help us:
- Decrease the tests times a lot
- Keep the tests time stable (my increasing the number of containers when they increase)