Add 'mock' to unit test dependencies for autogen libs. (#6402)

In anticipation of landing
googleapis/gapic-generator#2407.

Author: tseaver

packages/google-cloud-asset/noxfile.py

@@ -26,7 +26,7 @@
 
 def default(session):
     # Install all test dependencies, then install this package in-place.
-    session.install('pytest')
+    session.install('pytest', 'mock')
     for local_dep in LOCAL_DEPS:
         session.install('-e', local_dep)
     session.install('-e', '.')