Add tests for single_activator.0.dart API example.

Fixes name of the examples/api/lib/widgets/shortcuts/single_activator.single_activator.0.dart
Adds tests for examples/api/lib/widgets/shortcuts/single_activator.0.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.
[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.