TestGC, TestSameSourcePlayers, TestDeferredDeviceCreation,
TestPauseAndStopReading, TestPauseAndStopReadingWhilePaused,
TestPositionNotGrowingAfterFinished and
TestPlayerErrorDoesNotStopOtherPlayers now run in a synctest bubble, so
their sleeps and deadlines take no real time.
A bubble must not leave goroutines behind, so ResetContextForTesting
now stops the goroutine updating the players and the dummy players
reading their sources, and waits for them to exit. Before, every test
leaked them.
Updates #3716
dualfroz
d22b1a2b09