task idea: Use test helpers instead of hard-coding emails or calling get_user

[hackerkid]

• We trying to not hard code email address in tests. Instead of that we would use self.example_email, or self.mit_email or self.nonreg_email.
  eg: instead of iago@zulip.com we use self.example_email("iago")
• We also avoid calling get_user function in tests. Instead of that we use self.example_user or self.mit_user, self.nonreg_user or self.notification_bot.