Speed up clustered SQLTests

[purplefox]

A lot of the time spent in SQLTests is due to deleting all table data that occurs when a source or MV is dropped.

We could probably speed up SQLTests a lot by not actually deleting all data when a source/mv is dropped (only in tests) and then simply deleting all data at the end of the test run. We would also drop the test that no table data is left at end of run in SQLTests in this case.

For the normal test run we can use this optimisation and on the async integration test run we can do the delete for real (this will take longer).