Add tests for foreground notifications

[seadowg]

Follow up to #6900

This adds tests as well as fixing the bugs fixed in #6900 within the new structure.

Why is this the best possible solution? Were any other approaches considered?

Now, foreground service work manager jobs are run using real WorkManager infrastructure in tests. Deferred work still just runs on a standard background thread, but that seems like a fair trade-off for being able to control when they run still.

How does this change affect users? Describe intentional changes to behavior and behavior that could have accidentally been affected by code changes. In other words, what are the regression risks?

There's been some changes made to how background jobs work in general, so I think testing repeated jobs (like match exactly) as well as manually syncs (again with match exactly) is a good idea.

Before submitting this PR, please make sure you have:

• added or modified tests for any new or changed behavior
• run ./gradlew connectedAndroidTest (or ./gradlew testLab) and confirmed all checks still pass
• added a comment above any new strings describing it for translators
• added any new strings with date formatting to DateFormatsTest
• verified that any code or assets from external sources are properly credited in comments and/or in the about file.
• verified that any new UI elements use theme colors. UI Components Style guidelines