Remove semantics_tester import from cupertino/slider_test.dart (#184805)

Part of #182636

## Summary

Remove the `semantics_tester.dart` cross-import from
`cupertino/slider_test.dart`.

## What changed

* Replaced the deprecated `SemanticsTester`/`TestSemantics` assertion in
the slider semantics test with `tester.getSemantics(...)` and
`matchesSemantics(...)`
* Removed the `../widgets/semantics_tester.dart` import

## Testing

* `../../bin/flutter analyze test/cupertino/slider_test.dart`
* `../../bin/flutter test test/cupertino/slider_test.dart`

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Co-authored-by: Victor Sanni <victorsanniay@gmail.com>