test: use google-cloud-testutils (#25)

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Fixes #<issue_number_goes_here> 🦕

noxfile.py

@@ -110,8 +110,7 @@ def system(session):
 
     # Install all test dependencies, then install this package into the
     # virtualenv's dist-packages.
-    session.install("mock", "pytest")
-    session.install("git+https://github.com/googleapis/python-test-utils")
+    session.install("mock", "pytest", "google-cloud-testutils")
 
     session.install("-e", ".")