Add tests for shortcuts.dart API examples.

[ksokolovskyi]

This PR contributes to https://github.com/flutter/flutter/issues/130459

Description

• Adds tests for examples/api/lib/widgets/shortcuts/shortcuts.0.dart
• Adds tests for examples/api/lib/widgets/shortcuts/shortcuts.1.dart

Pre-launch Checklist

• [x] I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• [X] I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• [X] I read and followed the [Flutter Style Guide], including [Features we expect every widget to implement].
• [X] I signed the [CLA].
• [X] I listed at least one issue that this PR fixes in the description above.
• [ ] I updated/added relevant documentation (doc comments with ///).
• [X] I added new tests to check the change I am making, or this PR is [test-exempt].
• [X] I followed the [breaking change policy] and added [Data Driven Fixes] where supported.
• [X] All existing and new tests are passing.

[ksokolovskyi]

Hi @bleroux @TahaTesser thanks a lot for the review!

[ksokolovskyi]

@TahaTesser thanks for the replies and examples. I updated the tests according to your feedback. Could you please take a look again?

[ksokolovskyi]

@bleroux @TahaTesser thanks for the review!