test_classes: Use a valid (but reserved as fictional) phone number.

django-phonenumber-field 2.4.0 adds tighter phone number validation
that rejects +12223334444 for having an invalid area code.  This was
reverted in 4.0.0, but django-two-factor-auth still requires <3.99.

Signed-off-by: Anders Kaseorg <anders@zulipchat.com>

zerver/lib/test_classes.py

@@ -725,7 +725,7 @@ class ZulipTestCase(TestCase):
             yield
 
     def create_default_device(self, user_profile: UserProfile,
-                              number: str="+12223334444") -> None:
+                              number: str="+12125550100") -> None:
         phone_device = PhoneDevice(user=user_profile, name='default',
                                    confirmed=True, number=number,
                                    key='abcd', method='sms')