Add SliverAppBar showOnScreen semantics test coverage (#190410)

In https://github.com/flutter/flutter/pull/186611 the widgets test
`showOnScreen works with pinned app bar and sliver list` was switched
from `SliverAppBar` to a private `SliverPersistentHeader` delegate, as
part of removing the `material` import from
`scrollable_semantics_test.dart`. That was the only test exercising
`SemanticsAction.showOnScreen` against a pinned `SliverAppBar`, so the
Material path lost coverage and a TODO was left behind to track
restoring it.

This PR restores the coverage by adding an equivalent test to
`packages/flutter/test/material/app_bar_sliver_test.dart`, which already
imports `../widgets/semantics_tester.dart` and is where the other
`SliverAppBar` semantics tests live. The test performs
`SemanticsAction.showOnScreen` on the first list item and asserts it is
revealed *below* the pinned app bar (`dy == expandedHeight`) rather than
underneath it.

I also removed the TODO in
`packages/flutter/test/widgets/scrollable_semantics_test.dart`, since it
points at the issue this PR fixes. Note that the TODO says "in
material_ui package" — that package does not exist in the repo yet, so
the test is placed in the existing Material test suite (which matches
the issue description) and will move along with
`app_bar_sliver_test.dart` when the decoupling work lands. Happy to keep
the TODO instead if you'd prefer to track the `material_ui` move
separately.

Verification:
- `flutter test packages/flutter/test/material/app_bar_sliver_test.dart`
— 53 tests pass.
- `flutter test
packages/flutter/test/widgets/scrollable_semantics_test.dart` — 20 tests
pass.
- To confirm the assertion is load-bearing, flipping `pinned` to `false`
fails the new test with `Expected: <56.0> Actual: <6.0>`.

Fixes https://github.com/flutter/flutter/issues/189117

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we expect every widget to implement].
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- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.

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