In DEVPROD-7886 / DEVPROD-9487, we accidentally removed the piece of code in the plan cache fuzzer that takes a single generated query and varies the constants under it. This ticket is to undo this change, and also add sufficient fuzzer tests to keep this from happening again. For example, there should be a unit test ensuring that the plan cache fuzzer generates multiple queries from a single query shape, with varied constants where possible. Also, we should add the plan cache fuzzer to the self-test list so that it runs before merging.