While looking into resolving https://github.com/flutter/flutter/issues/117903, I found the massive test file `app_bar_test.dart` and found it unwieldy to work with. So before proposing a solution to #117903, which would touch many of these tests, I figured a clean up would be best first.

This splits up `app_bar_test.dart` with a new file `app_bar_sliver_test.dart`, and adds `app_bar_utils.dart` for shared test methods.

It basically moves all SliverAppBar tests into their own file, leaving just AppBar tests in the original file.