unit-tests: Prevent a failing worker thread to go wild after it fails

A worker raises SIGUSR1 to inform the main thread that the test fails. The main
thread then starts cancelling workers, but the offending thread should be
terminated immediately to prevent it from test continuation.

src/libstrongswan/tests/test_suite.c

@@ -136,7 +136,8 @@ static inline void test_failure()
 	else
 	{
 		pthread_kill(main_thread, SIGUSR1);
-		/* how can we stop just the thread? longjmp to a restore point? */
+		/* terminate thread to prevent it from going wild */
+		pthread_exit(NULL);
 	}
 }