[Notification scheduler]: Create MockNotificationBackgroundTaskScheduler

- Start working on all mock objects for scheduler.
- Moved this class under scheduler/tests.

Bug: 963304
Change-Id: I72ea3ed88028681acde31e4c80bfa8f095b22a74
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1716088
Commit-Queue: Hesen Zhang <hesen@chromium.org>
Reviewed-by: Xing Liu <xingliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#680243}