Fix a race in a test
startClient was written in such a way that it would leave a flush() running,
which could sometimes interfere with the rest of the test (or even subsequent
tests), causing sporadic test failures. Make sure that the flush completes
before we move on.
Fix a test which turned out to be relying on that behaviour (there was a flush
which ended up being a no-op, thus effectively inserting a pause allowing the
sync promise to complete.
Fix a beforeEach handler which was relying on startClient resolving to
undefined.
