ansible-test - Use `--forked` instead of `--boxed`

The `--boxed` option is deprecated.

changelogs/fragments/ansible-test-pytest-forked.yml

@@ -0,0 +1,2 @@
+bugfixes:
+  - ansible-test - Update unit tests to use the ``--forked`` option instead of the deprecated ``--boxed`` option.

test/lib/ansible_test/_data/requirements/units.txt

@@ -2,4 +2,5 @@ mock
 pytest
 pytest-mock
 pytest-xdist
+pytest-forked
 pyyaml  # required by the collection loader (only needed for collections)

test/lib/ansible_test/_internal/commands/units/__init__.py

@@ -237,7 +237,7 @@ def command_units(args):  # type: (UnitsConfig) -> None
     for test_context, python, paths, env in test_sets:
         cmd = [
             'pytest',
-            '--boxed',
+            '--forked',
             '-r', 'a',
             '-n', str(args.num_workers) if args.num_workers else 'auto',
             '--color',