test focus example 0

[derdilla]

Part of #130459. Adds tests for Focus example 0.

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.
• [X] 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.

[auto-submit[bot]]

auto label is removed for flutter/flutter/147564, due to - The status or check suite Google testing has failed. Please fix the issues identified (or deflake) before re-applying this label.

[derdilla]

@gspencergoog can you run Google testing again? I don't see how this could break anything.

[gspencergoog]

Done. It will work this time.