Regression tests: Evaluate regression test coverage for append and hash distributed tables

Citus originally only supported append distributed tables. So, we started building our regression tests for append distributed tables. We then added tests for hash distributed tables.

For example, two earlier regression tests in our test suite, `multi_create_table` and `multi_load_data` create append distributed tables. The following 30+ regression tests mostly operate on tables created in these regression tests.

We should evaluate our coverage for hash and append distributed tables. We could then update our regression tests to reflect Citus 7.x's real-world usage. A good starting point could be to count the number of regression tests that operate on append, hash, and range distributed tables.