[pointer_interceptor] fix integration test

[yjbanov]

The pointer interceptor test was probing into some implementation details of the web engine. As of flutter/engine#32712, the implementation has changed such that the semantics tree now sits on top of the platform views. This PR fixes the test accordingly.

Bonus:

• Test platform interceptor both with and without semantics enabled.
• Also test hit testing the background element directly (i.e. not just the buttons).

[ditman]

Thanks for the fix!

[ditman]

We should have picked this up with our roller, but tests for this package were being skipped. Running tests within the example directory of a package in CI is now being tracked here:

• [flutter_plugin_tools] Support example/test/ in test command flutter#102845

[fluttergithubbot]

This pull request is not suitable for automatic merging in its current state.

• The status or check suite publishable has failed. Please fix the issues identified (or deflake) before re-applying this label.

[ditman]

publishable was failing because this PR was modifying the main.dart of the example (which is published in pub.dev here). Bumped patch version so this PR can be published.