stripe tests: Set the active users explicitly.

Without this change, you could get obscure
failures when logging in as Cordelia if you
modified test data by doing something
fairly innocuous like adding a new test user.

Also the complicated query here to exclude
users was flaky, since it didn't explicitly
order by any field before doing the 'LIMIT 6'.

Part of the problem with debugging this flake
was that the failure would happen for the login,
but the data actually gets changed in `setUp,
which is easy to overlook, since it's not
explicitly invoked.

We continue to keep the seat_count set to
a constant, predictable value, since some
tests are very sensitive to having 6 users.

corporate/tests/test_stripe.py

@@ -223,18 +223,36 @@ class Kandra:  # nocoverage: TODO
 class StripeTestCase(ZulipTestCase):
     def setUp(self, *mocks: Mock) -> None:
         super().setUp()
-        # This test suite is not robust to users being added in populate_db. The following
-        # hack ensures get_latest_seat_count is fixed, even as populate_db changes.
         reset_emails_in_zulip_realm()
         realm = get_realm('zulip')
-        seat_count = get_latest_seat_count(realm)
-        assert(seat_count >= 6)
-        for user in UserProfile.objects.filter(realm=realm, is_active=True, is_bot=False) \
-                                       .exclude(role=UserProfile.ROLE_GUEST).exclude(email__in=[
+
+        # Explicitly limit our active users to 6 regular users,
+        # to make seat_count less prone to changes in our test data.
+        # We also keep a guest user and a bot to make the data
+        # slightly realistic.
+        active_emails = [
+            self.example_email('AARON'),
+            self.example_email('cordelia'),
             self.example_email('hamlet'),
-                                           self.example_email('iago')])[:seat_count-6]:
-            user.is_active = False
-            user.save(update_fields=['is_active'])
+            self.example_email('iago'),
+            self.example_email('othello'),
+            self.example_email('prospero'),
+            self.example_email('polonius'),  # guest
+            self.example_email('default_bot'),  # bot
+        ]
+
+        # Deactivate all users that aren't in our whitelist.
+        UserProfile.objects.exclude(email__in=active_emails).update(is_active=False)
+
+        # sanity check our 8 expected users are active
+        self.assertEqual(
+            UserProfile.objects.filter(realm=realm, is_active=True).count(),
+            8
+        )
+
+        # Our seat count excludes our guest user and bot, and
+        # we want this to be predictable for certain tests with
+        # arithmetic calculations.
         self.assertEqual(get_latest_seat_count(realm), 6)
         self.seat_count = 6
         self.signed_seat_count, self.salt = sign_string(str(self.seat_count))