[cupertino_ui] Migrate nav_bar_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 the skip annotation and custom SemanticsTester from nav_bar_test.dart, refactoring the semantics tests to use tester.ensureSemantics() and isSemantics. The review feedback suggests registering the disposal of SemanticsHandle using addTearDown immediately after creation to prevent resource leaks if assertions fail, and removing the redundant manual dispose calls.

[gemini-code-assist]

If the expect assertion fails, the test execution will halt immediately, and handle.dispose() at the end of the test will not be called. This leaves the global semantics state enabled, which can cause side effects or failures in subsequent tests. Registering the disposal with addTearDown immediately after creation ensures it is always cleaned up properly.

    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 handle.dispose() is now registered via addTearDown right after creation, this manual call at the end of the test is redundant and can be removed.

[Renzo-Olivares]

#11980 (comment)

[gemini-code-assist]

If the expect assertion fails, the test execution will halt immediately, and handle.dispose() at the end of the test will not be called. This leaves the global semantics state enabled, which can cause side effects or failures in subsequent tests. Registering the disposal with addTearDown immediately after creation ensures it is always cleaned up properly.

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

[Renzo-Olivares]

#11980 (comment)

[gemini-code-assist]

Since handle.dispose() is now registered via addTearDown right after creation, this manual call at the end of the test is redundant and can be removed.

[Renzo-Olivares]

#11980 (comment)

[gemini-code-assist]

If the expect assertion fails, the test execution will halt immediately, and handle.dispose() at the end of the test will not be called. This leaves the global semantics state enabled, which can cause side effects or failures in subsequent tests. Registering the disposal with addTearDown immediately after creation ensures it is always cleaned up properly.

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

[Renzo-Olivares]

#11980 (comment)

[gemini-code-assist]

Since handle.dispose() is now registered via addTearDown right after creation, this manual call at the end of the test is redundant and can be removed.

[Renzo-Olivares]

#11980 (comment)