tests: speed up retry conformance tests (#655)

* tests: speed up retry conf tests running in parallel

* update conftest readme

* update noxfile and comments

* check test session args

* lint

noxfile.py

@@ -154,24 +154,30 @@ def system(session):
 @nox.session(python=CONFORMANCE_TEST_PYTHON_VERSIONS)
 def conftest_retry(session):
     """Run the retry conformance test suite."""
-    conformance_test_path = os.path.join("tests", "conformance.py")
     conformance_test_folder_path = os.path.join("tests", "conformance")
-    conformance_test_exists = os.path.exists(conformance_test_path)
     conformance_test_folder_exists = os.path.exists(conformance_test_folder_path)
     # Environment check: only run tests if found.
-    if not conformance_test_exists and not conformance_test_folder_exists:
+    if not conformance_test_folder_exists:
         session.skip("Conformance tests were not found")
 
-    session.install("pytest",)
+    # Install all test dependencies and pytest plugin to run tests in parallel.
+    # Then install this package in-place.
+    session.install("pytest", "pytest-xdist")
     session.install("-e", ".")
 
+    # Run #CPU processes in parallel if no test session arguments are passed in.
+    if session.posargs:
+        test_cmd = [
+            "py.test",
+            "--quiet",
+            conformance_test_folder_path,
+            *session.posargs,
+        ]
+    else:
+        test_cmd = ["py.test", "-n", "auto", "--quiet", conformance_test_folder_path]
+
     # Run py.test against the conformance tests.
-    if conformance_test_exists:
-        session.run("py.test", "--quiet", conformance_test_path, *session.posargs)
-    if conformance_test_folder_exists:
-        session.run(
-            "py.test", "--quiet", conformance_test_folder_path, *session.posargs
-        )
+    session.run(*test_cmd)
 
 
 @nox.session(python=DEFAULT_PYTHON_VERSION)

tests/conformance/retry_conformance_testing.md → tests/conformance/README.md

@@ -27,3 +27,11 @@ To run the test suite locally:
 ```bash
 nox -s conftest_retry-3.8
 ```
+
+To run a single test locally:
+
+Single test names are displayed as "test-S{scenario_id}-{method}-{client-library-method-name}-{instructions index}", such as `test-S1-storage.buckets.get-bucket_reload-1`
+
+```bash
+nox -re conftest_retry-3.8 -- -k <single_test_name>
+```