Fix pytest warning when run via setup.py test (#524)

diff --git a/setup.py b/setup.py
index 9e593a8..adc1df5 100755
--- a/setup.py
+++ b/setup.py
@@ -40,7 +40,7 @@
 
     def finalize_options(self):
         test.test.finalize_options(self)
-        self.test_args = ['-x', "tests"]
+        self.test_args = ['-x', "tests/mobly"]
         self.test_suite = True
 
     def run_tests(self):