Fix flakiness in MT call tests

Using junit4 we can set a per-test timeout and then just do a blocking
wait until the setup is ready (instead of guessing and waiting n
milliseconds).

Bug: 78297012
Test: ran each test 100 times with no failures (previously saw failures
after 10-20 runs)
Change-Id: I77b106f52c90ef1d7a7928d60f4fed6600a8c685