Add possible message to flaky test check (elastic#8989)

TestCommands.test_modules_test_error relays on the presence of an error
message, this error message seems to be different under some
circumstances, as seen in CI builds.

Add the timeout message already seen, and print logs in case of failure
so it is more easy to fix if happens in the future.

metricbeat/tests/system/test_cmd.py

@@ -136,7 +136,15 @@ def test_modules_test_error(self):
             extra_args=["test", "modules"])
 
         assert exit_code == 0
-        assert self.log_contains("ERROR error making http request")
+        try:
+            assert any((
+                self.log_contains("ERROR error making http request"),
+                self.log_contains("ERROR timeout waiting for an event"),
+            ))
+        except:
+            # Print log to help debugging this if error message changes
+            print self.get_log()
+            raise
         assert self.log_contains("cpu...OK")
         assert self.log_contains("memory...OK")