autotest: read pexpects while waiting for sim time

If we do not drain pexpects regularly the ardupilot process can become blocked on terminal IO

Tools/autotest/common.py

@@ -2761,9 +2761,16 @@ class AutoTest(ABC):
     def get_sim_time(self, timeout=60):
         """Get SITL time in seconds."""
         self.drain_mav()
-        m = self.mav.recv_match(type='SYSTEM_TIME', blocking=True, timeout=timeout)
+        tstart = time.time()
-        if m is None:
+        while True:
+            self.drain_all_pexpects()
+            if time.time() - tstart > timeout:
                 raise AutoTestTimeoutException("Did not get SYSTEM_TIME message after %f seconds" % timeout)
+
+            m = self.mav.recv_match(type='SYSTEM_TIME', blocking=True, timeout=0.1)
+            if m is None:
+                continue
+
             return m.time_boot_ms * 1.0e-3
 
     def get_sim_time_cached(self):