Feat: Animate fill for material app bar
fixes: #162988
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Co-authored-by: Kate Lovett <katelovett@google.com>
Co-authored-by: LongCatIsLooong <31859944+LongCatIsLooong@users.noreply.github.com>
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.
