docs(python): pytest-xdist reference (#16231)

Partially resolves https://github.com/microsoft/playwright-python/issues/1442.

Tests: https://github.com/microsoft/playwright-pytest/pull/128.

docs/src/running-tests-python.md

@@ -47,6 +47,14 @@ You can run a single test, a set of tests or all tests. Tests can be run on one
   pytest test_login.py --browser webkit --browser firefox
   ```
 
+- Running Tests in parallel
+
+  ```bash
+  pytest --numprocesses auto
+  ```
+
+  (This assumes `pytest-xdist` is installed. For more information see [here](./test-runners.md#parallelism-running-multiple-tests-at-once).)
+
 For more information see [Playwright Pytest usage](./test-runners.md) or the Pytest documentation for [general CLI usage](https://docs.pytest.org/en/stable/usage.html).
 
 ## Running Tests

docs/src/test-runners-python.md

@@ -63,6 +63,21 @@ def test_my_app_is_working(fixture_name):
 - `browser_type_launch_args`: Override launch arguments for [`method: BrowserType.launch`]. It should return a Dict.
 - `browser_context_args`: Override the options for [`method: Browser.newContext`]. It should return a Dict.
 
+## Parallelism: Running Multiple Tests at Once
+
+If your tests are running on a machine with a lot of CPUs, you can speed up the overall execution time of your test suite by using [`pytest-xdist`](https://pypi.org/project/pytest-xdist/) to run multiple tests at once:
+
+```bash
+# install dependency
+pip install pytest-xdist
+# use the --numprocesses flag
+pytest --numprocesses auto
+```
+
+Depending on the hardware and nature of your tests, you can set `numprocesses` to be anywhere from `2` to the number of CPUs on the machine. If set too high, you may notice unexpected behavior.
+
+See [Running Tests](./running-tests.md) for general information on `pytest` options.
+
 ## Examples
 
 ### Configure Mypy typings for auto-completion