notifs [nfc]: Ensure tests pass with representative pm_users values.

To be reverted in the next commit.

The output of GroupPm.getPmUsersString in our Kotlin code was
actually a string of numbers separated by ', ' instead of ',' as we
expected. Ensure that our tests pass with ', ' before changing the
Kotlin code to use ','.

src/notification/__tests__/notification-test.js

@@ -30,7 +30,7 @@ describe('getNarrowFromNotificationData', () => {
   test('on notification for a group message returns a group narrow', () => {
     const notification = {
       recipient_type: 'private',
-      pm_users: '1,2,4',
+      pm_users: '1, 2, 4',
     };
     const usersById = new Map([
       [1, { email: '[email protected]' }],
@@ -47,7 +47,7 @@ describe('getNarrowFromNotificationData', () => {
   test('do not throw when users are not found; return null', () => {
     const notification = {
       recipient_type: 'private',
-      pm_users: '1,2,4',
+      pm_users: '1, 2, 4',
     };
     const usersById = new Map();