Add unittests for unnotification_builder_mac

This CL introduces unittests that test the UNNotification builder which
will be added to when the builder sees more updates.

Bug: None
Change-Id: I8c52cb2f5b9641dfc807ace6d9b0c8ea01eecbf9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2440629
Reviewed-by: Rayan Kanso <rayankans@chromium.org>
Reviewed-by: Peter Beverloo <peter@chromium.org>
Reviewed-by: Richard Knoll <knollr@chromium.org>
Commit-Queue: Mohamed Adel <adelm@google.com>
Cr-Commit-Position: refs/heads/master@{#813173}