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.

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):