[cupertino_ui] Migrate checkbox_test.dart to SemanticsHandle

[QuncCccccc]

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

This PR:

• Removes the cross-import of widgets/semantics_tester.dart from checkbox_test.dart.
• Replaces the remaining SemanticsTester usages with SemanticsHandle and direct tester.getSemantics expectations.
• Removes the @Skip annotation and moves the file to the 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 checkbox_test.dart and refactors the semantics tests to use tester.ensureSemantics() and matchesSemantics instead of SemanticsTester. The reviewer feedback recommends replacing the fragile find.byType(Focus).last finder with a more robust scoped descendant finder to target the Focus widget associated with CupertinoCheckbox.

[chunhtai]

This is not needed

[chunhtai]

same

[justinmc]

LGTM minus @chunhtai's point about ensureSemantics 👍