testsuite/smokey: posix-mutex: Expand mutex holding time when running in VM

When running in QEMU, the 5 ms holding time of mutex_timed_locker may be
too short when the host is busy. This occasionally leads to failing
tests in our CI:

posix-mutex.c:510, FAILED: get_effective_prio() == prio (=false)

Expand to 50 ms in such setups.
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

testsuite/smokey/posix-mutex/posix-mutex.c

@@ -170,7 +170,8 @@ static void *mutex_timed_locker(void *arg)
 	int ret;
 
 	clock_gettime(CLOCK_REALTIME, &now);
-	timespec_adds(&ts, &now, 5000000); /* 5ms from now */
+	/* 5ms (or 50ms in VM) from now */
+	timespec_adds(&ts, &now, smokey_on_vm ? 50000000 : 5000000);
 
 	if (p->barrier)
 		smokey_barrier_release(p->barrier);