chore(generator): support parallel system test execution in generated noxfiles

[ohmayr]

Updates the default noxfile.py template (noxfile.py.j2) in gapic-generator to support parallel test execution for system test suites by default using pytest-xdist. This allows all generated python client library packages to run their system tests in parallel.

• Template Updates:
  • Added "pytest-xdist" to SYSTEM_TEST_STANDARD_DEPENDENCIES in gapic/templates/noxfile.py.j2.
  • Updated py.test invocations in the system(session) session of noxfile.py.j2 to run with parallel worker allocation (-n auto).

[gemini-code-assist]

Code Review

This pull request introduces parallel execution for system tests by adding pytest-xdist to the dependencies and configuring pytest to run with -n auto in the noxfile.py.j2 template and its generated golden files. The reviewer recommends making the number of parallel workers configurable via an environment variable (e.g., NOX_SYSTEM_WORKERS) instead of hardcoding auto to prevent potential flakiness, race conditions, or API rate-limiting issues when interacting with live Google Cloud resources.

[ohmayr]

I am going to hold this one off until we can test the changes across all the packages that have system tests.

[daniel-sanche]

I wonder if this could cause failures in some packages? Not all tests are written with concurrency in mind. And I could imagine some backends may have limitations too