Replace instances of reactor pumping with get_success. (#7619)

Calls `self.get_success` on all deferred methods instead of abusing `self.pump()`. This has the benefit of working with coroutines, as well as checking that method execution completed successfully.

There are also a few small cleanups that I made in the process.

Author: anoadragon453

changelog.d/7619.misc

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+Check that all asynchronous tasks succeed and general cleanup of `MonthlyActiveUsersTestCase` and `TestMauLimit`.