Firestore: Improve cleanups for 'watch' system tests. (#8638)

H/t to @pchauhan-qlogic for identifying the need to unsubscribe.

Also, use unique collection IDs for 'watch' tests.

Exception for 'test_watch_query_order', which requires that we use the
special 'users' collection, in order to take advantage of its index.

Supersedes #8497.

Closes #6605, #7130.