[cupertino_ui] Migrate button_test.dart to SemanticsHandle

[Renzo-Olivares]

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

This PR:

• Removed the cross-import of widgets/semantics_tester.dart. Replaced SemanticsTester with SemanticsHandle.
• Removed @Skip annotation, all tests in this file has passed. semantics_tester.dart has existed in cupertino_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 a skip annotation from button_test.dart and refactors the Cupertino button semantics test to use standard Flutter semantics testing utilities instead of a custom helper. The review feedback suggests using addTearDown to ensure the SemanticsHandle is properly disposed of even if assertions fail, allowing the manual disposal call to be removed.

[gemini-code-assist]

To ensure that the SemanticsHandle is always disposed of, even if an assertion/expectation fails during the test, it is recommended to use addTearDown(handle.dispose) immediately after creating it.

Suggested change

	final SemanticsHandle handle = tester.ensureSemantics();
	final SemanticsHandle handle = tester.ensureSemantics();
	addTearDown(handle.dispose);

[Renzo-Olivares]

This is how the test was previously structured, no addTearDown was previously used. Also explanation from gemini below as to why the test fails if we do use addTearDown(handle.dispose).

From gemini:

Flutter's TestWidgetsFlutterBinding runs its end-of-test leak verifications (specifically _verifySemanticsHandlesWereDisposed) inside the test body execution flow, immediately after your test callback completes, but before returning control to the Dart test runner:

1. [Test Runner] Starts test
2.   [Flutter Binding] Runs test body (your async callback)
3.     Your test code executes...
4.     Your test code completes.
5.   [Flutter Binding] Runs _endOfTestVerifications()  <-- CRITICAL POINT
6.     Checking active SemanticsHandles... 
       ERROR: "A SemanticsHandle was active at the end of the test."
7. [Test Runner] Runs addTearDown callbacks            <-- TOO LATE!
8.   handle.dispose() is called here.

Because the verification (Step 5) happens before the test runner runs the teardown callbacks (Step 7), the binding sees that the SemanticsHandle is still active and throws a FlutterError, failing the test.

[gemini-code-assist]

Since addTearDown(handle.dispose) is now used to clean up the SemanticsHandle, this manual call to handle.dispose() can be removed.

[Renzo-Olivares]

#11992 (comment)

[Piinks]

LGTM