Use py.test from run_tests.py

Also updates the version of grpcio_test. Now that grpcio and its tests
are in different project packages, the version numbers should be kept in
sync.
diff --git a/src/python/grpcio_test/commands.py b/src/python/grpcio_test/commands.py
index 24d6241..c796d94 100644
--- a/src/python/grpcio_test/commands.py
+++ b/src/python/grpcio_test/commands.py
@@ -52,4 +52,6 @@
     # We import here to ensure that setup.py has had a chance to install the
     # relevant package eggs first.
     import pytest
-    pytest.main(self.pytest_args)
+    result = pytest.main(self.pytest_args)
+    if result != 0:
+      raise SystemExit(result)