Clearer scenario names in tests

Only after finding [these docs](https://github.com/devcontainers/cli/blob/main/docs/features/test.md#scenarios) did I understand that this `hello`:

https://github.com/devcontainers/feature-starter/blob/67a45f05bb47843fef96f53ab09eab04680f6d95/test/hello/scenarios.json#L5

refers to the actual feature name, but this `hello`:

https://github.com/devcontainers/feature-starter/blob/67a45f05bb47843fef96f53ab09eab04680f6d95/test/hello/scenarios.json#L2

refers to the `hello` in the filename for [`hello.sh`](https://github.com/devcontainers/feature-starter/blob/main/test/hello/hello.sh) and not the command itself.

Okay with a PR to clarify and perhaps link to the test docs?