Use existing TestScope instead of creating a new one

[veyndan]

I'm trying to see if I can elegantly remove the dependence of PagingDataDifferTest.kt on MainDispatcherRule, without just inlining the class. Keeping track of all the CoroutineScopes for each test is difficult though, so I'm trying to slightly reduce it here (i.e., use the TestScope that the SimpleDiffer is invoked from, instead of creating a new one).

Test: ./gradlew test connectedCheck

[veyndan]

I managed to remove the usage of MainDispatcherRule at #562. This change is no longer required. Personally, I find this change easier to read than what was before, but if you disagree, feel free to just close this PR.

[claraf3]

Thanks for the clean up!