chore: make doctest tests less parallel (#960)

Parallel test runs contribute
to them running into rate limiting quota issue with cloud functions.
Currently doctest nox session is running in about 9 minutes, while
presubmit is running in 45 minutes. This gives us some leeway in making
doctest less parallel to gain less likelihood of running into rate quota
issue without comprimising the overall PR merge readiness turnaround.

Author: shobsi

noxfile.py

@@ -304,6 +304,7 @@ def run_system(
     print_duration=False,
     extra_pytest_options=(),
     timeout_seconds=900,
+    num_workers=20,
 ):
     """Run the system test suite."""
     constraints_path = str(
@@ -323,7 +324,7 @@ def run_system(
     pytest_cmd = [
         "py.test",
         "--quiet",
-        "-n=20",
+        f"-n={num_workers}",
         # Any individual test taking longer than 15 mins will be terminated.
         f"--timeout={timeout_seconds}",
         # Log 20 slowest tests
@@ -392,6 +393,7 @@ def doctest(session: nox.sessions.Session):
         ),
         test_folder="bigframes",
         check_cov=True,
+        num_workers=5,
     )