Some of the tests in ServiceStateMachineTest schedule tasks on ServiceExecutorFixed (e.g., ServiceStateMachineWithBorrowedThreadsTest). This is to run the tasks scheduled by ServiceStateMachine on the fixed thread-pool (see here).

Since we don't currently wait for the fixed thread-pool to join before destroying the ServiceStateMachineTest test-fixture (see here), there's a possibility to destroy the service context before destroying all clients, thus hitting this invariant invariant(_clients.empty());.

Acceptance criteria: retire all tasks on fixed thread-pool before calling the destructor for ServiceStateMachineTest.