Refs #26351 -- Added check hook to support database-related checks

Thanks Tim Graham and Shai Berger for the reviews.

tests/check_framework/test_database.py

@@ -0,0 +1,33 @@
+import unittest
+
+from django.core.checks import Tags, run_checks
+from django.core.checks.registry import CheckRegistry
+from django.db import connection
+from django.test import TestCase, mock
+
+
+class DatabaseCheckTests(TestCase):
+    @property
+    def func(self):
+        from django.core.checks.database import check_database_backends
+        return check_database_backends
+
+    def test_database_checks_not_run_by_default(self):
+        """
+        `database` checks are only run when their tag is specified.
+        """
+        def f1(**kwargs):
+            return [5]
+
+        registry = CheckRegistry()
+        registry.register(Tags.database)(f1)
+        errors = registry.run_checks()
+        self.assertEqual(errors, [])
+
+        errors2 = registry.run_checks(tags=[Tags.database])
+        self.assertEqual(errors2, [5])
+
+    def test_database_checks_called(self):
+        with mock.patch('django.db.backends.base.validation.BaseDatabaseValidation.check') as mocked_check:
+            run_checks(tags=[Tags.database])
+            self.assertTrue(mocked_check.called)