autotest: pause/unpause SITL while draining mav

If Python can't keep up with the message volume coming from the autopilot we never manage to drain all messages from the vehicle.

So try pausing/unpausing the simulation so we can drain the link...

AT-1968.6: AP: PreArm: Radio failsafe on
AT-1969.9: AP: PreArm: Radio failsafe on
AT-1971.2: AP: PreArm: Radio failsafe on
AT-1972.4: AP: PreArm: Radio failsafe on
AT-1973.7: AP: PreArm: Radio failsafe on
AT-1974.9: AP: PreArm: Radio failsafe on
AT-1975.3: Drained 2000283 messages from mav (7218.974791/s)
AT-1975.3: Exception caught: Traceback (most recent call last):
  File "/mnt/volume_nyc3_01/autotest/APM/APM/Tools/autotest/common.py", line 699
8, in run_one_test_attempt
    self.context_pop()
  File "/mnt/volume_nyc3_01/autotest/APM/APM/Tools/autotest/common.py", line 499
3, in context_pop
    self.set_parameters(dead_parameters_dict, add_to_context=False)

Tools/autotest/common.py

@@ -14,6 +14,7 @@ import math
 import os
 import re
 import shutil
+import signal
 import sys
 import time
 import traceback
@@ -2558,6 +2559,16 @@ class AutoTest(ABC):
         self.apply_default_parameters()
         self.progress("Reset SITL commandline to default")
 
+    def pause_SITL(self):
+        '''temporarily stop the SITL process from running.  Note that
+        simulation time will not move forward!'''
+        # self.progress("Pausing SITL")
+        self.sitl.kill(signal.SIGSTOP)
+
+    def unpause_SITL(self):
+        # self.progress("Unpausing SITL")
+        self.sitl.kill(signal.SIGCONT)
+
     def stop_SITL(self):
         self.progress("Stopping SITL")
         self.expect_list_remove(self.sitl)
@@ -2737,6 +2748,7 @@ class AutoTest(ABC):
         tstart = time.time()
         timeout = 120
         failed_to_drain = False
+        self.pause_SITL()
         while mav.recv_msg() is not None:
             count += 1
             if time.time() - tstart > timeout:
@@ -2745,6 +2757,7 @@ class AutoTest(ABC):
                 # just die if that seems to be the case:
                 failed_to_drain = True
                 quiet = False
+        self.unpause_SITL()
         if quiet:
             self.in_drain_mav = False
             return