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/.gitignore b/.gitignore
index 89bd100..f5ca501 100644
--- a/.gitignore
+++ b/.gitignore
@@ -33,3 +33,6 @@
 
 # Makefile's cache
 cache.mk
+
+# Temporary test reports
+report.xml