Fixed #36083 -- Ran system checks in ParallelTestSuite workers.

Workers created by ParallelTestSuite were not running system
checks in the spawn multiprocessing mode. In general this is
fine, but system checks can have side effects expected by tests.

This patch runs system checks inside of _init_worker, which is
only called by ParallelTestSuite.

django/test/runner.py

@@ -439,7 +439,7 @@ def _init_worker(
     used_aliases=None,
 ):
     """
-    Switch to databases dedicated to this worker.
+    Switch to databases dedicated to this worker and run system checks.
 
     This helper lives at module-level because of the multiprocessing module's
     requirements.
@@ -463,6 +463,9 @@ def _init_worker(
             process_setup(*process_setup_args)
         django.setup()
         setup_test_environment(debug=debug_mode)
+        call_command(
+            "check", stdout=io.StringIO(), stderr=io.StringIO(), databases=used_aliases
+        )
 
     db_aliases = used_aliases if used_aliases is not None else connections
     for alias in db_aliases:

tests/test_runner/models.py

@@ -6,6 +6,13 @@ class Person(models.Model):
     last_name = models.CharField(max_length=20)
     friends = models.ManyToManyField("self")
 
+    system_check_run_count = 0
+
+    @classmethod
+    def check(cls, *args, **kwargs):
+        cls.system_check_run_count += 1
+        return super().check(**kwargs)
+
 
 # A set of models that use a non-abstract inherited 'through' model.
 class ThroughBase(models.Model):

tests/test_runner/test_parallel.py

@@ -8,6 +8,8 @@ from unittest.suite import TestSuite, _ErrorHolder
 from django.test import SimpleTestCase
 from django.test.runner import ParallelTestSuite, RemoteTestResult
 
+from . import models
+
 try:
     import tblib.pickling_support
 except ImportError:
@@ -48,6 +50,9 @@ class ParallelTestRunnerTest(SimpleTestCase):
             with self.subTest(index=i):
                 self.assertEqual(i, i)
 
+    def test_system_checks(self):
+        self.assertEqual(models.Person.system_check_run_count, 1)
+
 
 class SampleFailingSubtest(SimpleTestCase):
     # This method name doesn't begin with "test" to prevent test discovery