[cupertino_ui] Create util files. Remove widgets import in adaptive_text_selection_toolbar_test.dart and text_selection_test.dart

[QuncCccccc]

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

This PR:

• Removed the cross-import of widgets/clipboard_utils.dart and widgets/text_selection_toolbar_utils.dart.
• Removed @Skip tag, all tests in this file has passed. Created util files for clipboard_utils.dart and text_selection_toolbar_utils.dart
• Moved the files to test/ folder.

This is to port changes in flutter/flutter#184278

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• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
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• 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 resolves cross-import issues by removing @Skip annotations from adaptive_text_selection_toolbar_test.dart and text_selection_test.dart, and introducing local test utilities clipboard_utils.dart and text_selection_toolbar_utils.dart. Feedback is provided regarding a potential runtime TypeError in MockClipboard when casting clipboardData to Map<String, dynamic>? instead of Map<dynamic, dynamic>?.

[gemini-code-assist]

Casting clipboardData directly to Map<String, dynamic>? can throw a TypeError at runtime. When platform channel methods like Clipboard.setData are invoked, the arguments are decoded as Map<Object?, Object?> (or Map<dynamic, dynamic>) by the standard message codec. In Dart, a Map<Object?, Object?> cannot be cast to Map<String, dynamic> using as and will result in a TypeError. Using Map<dynamic, dynamic>? is safer and avoids this issue.

Suggested change

	final clipboardDataMap = clipboardData as Map<String, dynamic>?;
	final clipboardDataMap = clipboardData as Map<dynamic, dynamic>?;

[Piinks]

LGTM!