subdomains: Write some tests for get_subdomain.

This logic is a bit subtle, and we're about to make changes to it.
So let's have some tests.

zerver/tests/test_subdomains.py

@@ -0,0 +1,31 @@
+
+import mock
+from typing import Any, List
+
+from django.test import TestCase, override_settings
+
+from zerver.lib.subdomains import get_subdomain
+from zerver.models import Realm
+
+class SubdomainsTest(TestCase):
+    def test_get_subdomain(self):
+        # type: () -> None
+
+        def request_mock(host):
+            # type: (str) -> Any
+            request = mock.Mock(spec=['get_host'])
+            request.attach_mock(mock.Mock(return_value=host), 'get_host')
+            return request
+
+        def test(expected, host, *, root_aliases=[]):
+            # type: (str, str, List[str]) -> None
+            with self.settings(EXTERNAL_HOST='example.org',
+                               ROOT_SUBDOMAIN_ALIASES=root_aliases):
+                self.assertEqual(get_subdomain(request_mock(host)), expected)
+                self.assertEqual(get_subdomain(request_mock(host + ':443')), expected)
+
+        ROOT = Realm.SUBDOMAIN_FOR_ROOT_DOMAIN
+        test(ROOT, 'example.org')
+        test('foo', 'foo.example.org')
+        test(ROOT, 'www.example.org', root_aliases=['www'])
+        test(ROOT, 'arbitrary.com')