fix(python_library): fix unit test dependency installation (#811)

Perviously, system tests allow users to pass in separate lists for `system_test_external_dependencies` and `unit_test_local_dependencies`, which would install dependencies needed for the test. Unit tests only had a single option called `unit_test_dependencies`, which only supported local packages.

This PR makes unit tests consistent with system tests by handling `unit_test_external_dependencies` and `unit_test_local_dependencies`. I kept  `unit_test_dependencies` around with it's previous behaviour for backwards compatibility, in case any repos are using it as-is.

synthtool/gcp/templates/python_library/noxfile.py.j2

@@ -76,9 +76,10 @@ def default(session):
     {%- if microgenerator %}
     session.install("asyncmock", "pytest-asyncio")
     {% endif %}
-    session.install("mock", "pytest", "pytest-cov")
-    session.install("-e", "."){% for dependency in unit_test_dependencies %}
-    session.install("-e", "{{dependency}}"){% endfor %}
+    session.install("mock", "pytest", "pytest-cov" {% for d in unit_test_external_dependencies %}"{{d}}"{% if not loop.last %},{% endif %}{% endfor %})
+    session.install("-e", ".")
+    {% for dependency in unit_test_local_dependencies %}session.install("-e", "{{dependency}}"){% endfor %}
+    {% for dependency in unit_test_dependencies %}session.install("-e", "{{dependency}}"){% endfor %}
 
     # Run py.test against the unit tests.
     session.run(