chore(deps): upgrade mailparser to 3.x in auth-server test mail helper
Because: * mailparser 0.6.1 is old, and only the auth test mail helper uses it * the 3.x parser is async and returns a different mail shape This commit: * upgrades mailparser to 3.x in fxa-auth-server * ports mail_helper to simpleParser and keeps the 0.x mail shape * copies messageId into the legacy shape for the resend specs * stores mail in arrival order and accepts only after the store * times out or rejects a bad mail so later mail is not stalled Closes FXA-5351