tests: Make sure import doesn't corrupt original realm.

The original intention of this was to prevent coding
errors with realm getters that don't, um, filter
on realm.

Unfortunately, you can still write a broken realm getter
that forgets to filter on realm, but which returns a
Set, and the new safeguards won't see any difference.

We could make all the getters return sorted lists
instead, but that's for another day.

This code does serve another purpose, which is to
prevet egregious bugs in the import itself.

zerver/tests/test_import_export.py

@@ -795,12 +795,33 @@ class ImportExportTest(ZulipTestCase):
         realm_user_default.twenty_four_hour_time = True
         realm_user_default.save()
 
+        getters = self.get_realm_getters()
+
+        snapshots: Dict[str, Any] = {}
+
+        for f in getters:
+            snapshots[f.__name__] = f(original_realm)
+
         self._export_realm(original_realm)
 
         with self.settings(BILLING_ENABLED=False), self.assertLogs(level="INFO"):
             do_import_realm(os.path.join(settings.TEST_WORKER_DIR, "test-export"), "test-zulip")
 
-        getters = self.get_realm_getters()
+        # Make sure our export/import didn't somehow leak info into the
+        # original realm.
+        for f in getters:
+            # One way this will fail is if you make a getter that doesn't
+            # properly restrict its results to a single realm.
+            if f(original_realm) != snapshots[f.__name__]:
+                raise AssertionError(
+                    f"""
+                    The export/import process is corrupting your
+                    original realm according to {f.__name__}!
+
+                    If you wrote that getter, are you sure you
+                    are only grabbing objects from one realm?
+                    """
+                )
 
         imported_realm = Realm.objects.get(string_id="test-zulip")