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

Thank you for opening a Pull Request! Before submitting your PR, there are a few things you can do to make sure it goes smoothly:
- [ ] Make sure to open an issue as a [bug/issue](https://github.com/googleapis/python-asset/issues/new/choose) before writing your code!  That way we can discuss the change, evaluate designs, and agree on the general idea
- [ ] Ensure the tests and linter pass
- [ ] Code coverage does not decrease (if any source code was changed)
- [ ] Appropriate docs were updated (if necessary)

Fixes #<issue_number_goes_here> 🦕

Author: busunkim96

packages/google-cloud-asset/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", ".")