Fixed #16905 -- Added extensible checks (nee validation) framework

This is the result of Christopher Medrela's 2013 Summer of Code project.

Thanks also to Preston Holmes, Tim Graham, Anssi Kääriäinen, Florian
Apolloner, and Alex Gaynor for review notes along the way.

Also: Fixes #8579, fixes #3055, fixes #19844.

tests/proxy_model_inheritance/tests.py

@@ -5,6 +5,7 @@ import sys
 
 from django.core.management import call_command
 from django.test import TestCase, TransactionTestCase
+from django.test.utils import override_system_checks
 from django.utils._os import upath
 
 from .models import (ConcreteModel, ConcreteModelSubclass,
@@ -26,6 +27,9 @@ class ProxyModelInheritanceTests(TransactionTestCase):
     def tearDown(self):
         sys.path = self.old_sys_path
 
+    # `auth` app is imported, but not installed in this test, so we need to
+    # exclude checks registered by this app.
+    @override_system_checks([])
     def test_table_exists(self):
         with self.modify_settings(INSTALLED_APPS={'append': ['app1', 'app2']}):
             call_command('migrate', verbosity=0)