test: Use random email in eg.user

This specifically fixes a runtime error from the test
"find account among several" in
`test/notifications/display_test.dart` due to duplicate email
addresses within a realm.  This also useful in general in cases
when the email addresses do not matter and should be kept boring.

The generation approach is similar to an ID, to make collision
unlikely (still possible with caller specified values).

Signed-off-by: Zixuan James Li <zixuan@zulip.com>

Author: PIG208

test/example_data.dart

@@ -70,13 +70,21 @@ GetServerSettingsResult serverSettings({
 int _nextUserId() => (_lastUserId += 1 + Random().nextInt(100));
 int _lastUserId = 1000;
 
+/// A random email address, unique from previously generated ones.
+String _nextEmail() => 'mail${_lastEmailSuffix += Random().nextInt(1000)}@example.com';
+int _lastEmailSuffix = 1000;
+
 /// Construct an example user.
 ///
 /// If user ID `userId` is not given, it will be generated from a random
 /// but increasing sequence.
 /// Use an explicit `userId` only if the ID needs to correspond to some
 /// other data in the test, or if the IDs need to increase in a different order
 /// from the calls to [user].
+///
+/// If `deliveryEmail` is not given, it will be generated from a random
+/// but unique sequence of strings.
+/// If `email` is not given, it will be set to `deliveryEmail`.
 User user({
   int? userId,
   String? deliveryEmail,
@@ -87,7 +95,7 @@ User user({
   String? avatarUrl,
   Map<int, ProfileFieldUserData>? profileData,
 }) {
-  var effectiveDeliveryEmail = deliveryEmail ?? 'name@example.com'; // TODO generate example emails
+  var effectiveDeliveryEmail = deliveryEmail ?? _nextEmail();
   _checkPositive(userId, 'user ID');
   return User(
     userId: userId ?? _nextUserId(),