[material_ui] Remove widgets import in slider_test.dart

[QuncCccccc]

Part of flutter/flutter#182636 and flutter/flutter#188395

This PR:

• Removed the cross-import of widgets/semantics_tester.dart.
• Removed @Skip tag, all tests in this file has passed. semantics_tester.dart has existed in material_ui, so we can directly import semantics_tester.dart;
• Moved the file to test/ folder.

Pre-Review Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I read the [AI contribution guidelines] and understand my responsibilities, or I am not using AI tools.
• I read the [Tree Hygiene] page, which explains my responsibilities.
• I read and followed the [relevant style guides] and ran [the auto-formatter].
• I signed the [CLA].
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I [linked to at least one issue that this PR fixes] in the description above.
• I followed [the version and CHANGELOG instructions], using [semantic versioning] and the [repository CHANGELOG style], or I have commented below to indicate which documented exception this PR falls under[^1].
• I updated/added any relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or I have commented below to indicate which [test exemption] this PR falls under[^1].
• All existing and new tests are passing.

[gemini-code-assist]

Code Review

This pull request removes the @Skip annotation from packages/material_ui/test/slider_test.dart, reorders the package imports, and updates the import path for semantics_tester.dart to a local import. There are no review comments, and I have no feedback to provide.

[Piinks]

LGTM