py/scheduler: Add assert that scheduler is locked when unlocking.

And add a test that shows how this can happen when multiple threads are
accessing the scheduler, which fails if atomic sections are not used.

py/scheduler.c

@@ -108,6 +108,7 @@ void mp_sched_lock(void) {
 
 void mp_sched_unlock(void) {
     mp_uint_t atomic_state = MICROPY_BEGIN_ATOMIC_SECTION();
+    assert(MP_STATE_VM(sched_state) < 0);
     if (++MP_STATE_VM(sched_state) == 0) {
         // vm became unlocked
         if (MP_STATE_VM(mp_pending_exception) != MP_OBJ_NULL || mp_sched_num_pending()) {