autotest: drain pexpects in set_parameters loop

This loop doesn't self.mav.recv(), so the idle loop isn't called, so the
pexpects aren't drained.  That can cause ArduPilot to block on stderr if
it is trying to print a stacktrace, for example

Tools/autotest/common.py

@@ -4032,6 +4032,7 @@ class AutoTest(ABC):
         autopilot_values = {}
         for i in range(attempts):
             self.drain_mav(quiet=True)
+            self.drain_all_pexpects()
             received = set()
             for (name, value) in want.items():
                 if verbose: