autotest: loosen constrain on proximity distance message

the floating/double changes appear to have cause this to start to flap

Tools/autotest/arducopter.py

@@ -7137,6 +7137,7 @@ def ProximitySensors(self):
             failed = False
             wants = []
             gots = []
+            epsilon = 20
             while True:
                 if self.get_sim_time_cached() - tstart > 30:
                     raise AutoTestTimeoutException("Failed to get distances")
@@ -7149,7 +7150,7 @@ def ProximitySensors(self):
                 want = expected_distances_copy[m.orientation]
                 wants.append(want)
                 gots.append(got)
-                if abs(want - got) > 5:
+                if abs(want - got) > epsilon:
                     failed = True
                 del expected_distances_copy[m.orientation]
             if failed: