Trac #33825: Use pytest-xdist to run pytest in parallel

(from https://trac.sagemath.org/ticket/33546#comment:13 -
https://trac.sagemath.org/ticket/33546#comment:20)

Use https://github.com/pytest-dev/pytest-xdist to run pytest in
parallel.

URL: https://trac.sagemath.org/33825
Reported by: mkoeppe
Ticket author(s): Tobias Diez
Reviewer(s): Dima Pasechnik, Matthias Koeppe

build/pkgs/pytest/dependencies

@@ -1,4 +1,4 @@
-$(PYTHON) pluggy packaging attrs py pyparsing importlib_metadata | $(PYTHON_TOOLCHAIN)
+$(PYTHON) pluggy packaging attrs py pyparsing importlib_metadata tomli | $(PYTHON_TOOLCHAIN)
 
 ----------
 All lines of this file are ignored except the first.

build/pkgs/pytest_xdist/SPKG.rst

@@ -0,0 +1,18 @@
+pytest_xdist: pytest xdist plugin for distributed testing and loop-on-failing modes
+===================================================================================
+
+Description
+-----------
+
+pytest xdist plugin for distributed testing and loop-on-failing modes
+
+License
+-------
+
+MIT
+
+Upstream Contact
+----------------
+
+https://pypi.org/project/pytest-xdist/
+

build/pkgs/pytest_xdist/dependencies

@@ -0,0 +1,4 @@
+$(PYTHON) pytest | $(PYTHON_TOOLCHAIN)
+
+----------
+All lines of this file are ignored except the first.

build/pkgs/pytest_xdist/distros/conda.txt

@@ -0,0 +1 @@
+pytest-xdist

build/pkgs/pytest_xdist/requirements.txt

@@ -0,0 +1 @@
+pytest-xdist

build/pkgs/pytest_xdist/type

@@ -0,0 +1 @@
+optional

src/doc/en/developer/tools.rst

@@ -252,13 +252,18 @@ package :mod:`sage.numerical.backends` and some modules in
 
 *Installation:*
 
-- ``./sage -i pytest``.
+- ``./sage -i pytest pytest_xdist``.
 
 *Usage:*
 
 - Tox, Sage doctester: At the end of ``./sage -t`` (or ``./sage --tox -e doctest``), Pytest is automatically invoked.
 
-- Manual: Run ``./sage -pytest path/to/the/test_file.py`` or ``./sage -pytest`` to run all tests.
+- Manual: Run ``./sage -pytest path/to/the/test_file.py`` or ``./sage -pytest``
+  to run all tests. The additional argument ``-n`` can be used to
+  distribute tests across multiple CPUs to speed up test execution.
+  For example, ``./sage -pytest -n 4`` will run 4 tests in parallel, while
+  ``./sage -pytest -n auto`` will spawn a number of workers processes equal
+  to the number of available CPUs.
 
 - VS Code: Install the `Python extension <https://marketplace.visualstudio.com/items?itemName=ms-python.python>`_ and follow the `offical VS Code documentation <https://code.visualstudio.com/docs/python/testing>`__.