chore: parallelize presubmit unit test runs with pytest-xdist

PiperOrigin-RevId: 823199807

Author: speedstorm1

noxfile.py

@@ -205,6 +205,8 @@ def default(session):
     session.run(
         "py.test",
         "--quiet",
+        "-n",
+        "auto",
         f"--junitxml=unit_{session.python}_sponge_log.xml",
         "--cov=google",
         "--cov-append",