Make it easier to scaffold tests within a plugin

[swissspidy]

Feature Request

• [x] Yes, I reviewed the contribution guidelines.

Describe your use case and the problem you are facing

Adding tests for commands within a plugin requires a bit more work at the moment. It would be nice if we could make that a bit easier.

See https://github.com/10up/plugin-check/pull/215 for a recent example, see also the code coverage part and the custom behat config and FeatureContext.php file.

Describe the solution you'd like

Add a new arg to the scaffold-package-tests command to indicate that the tests are for a specific plugin (or detect whether it's a plugin folder) and change logic accordingly.