Fail build when scaffolded README is out of date

[danielbachhuber]

In our default scaffolded bin/test.sh, we should have some feature for failing the build when the scaffolded README is out of date.

Functionally, we'd test this by running wp package install wp-cli/scaffold-package-command && wp scaffold package-readme ./ --force, and then erring if any README changes were detected.

However, we'll want to be respectful of packages that only use scaffold package-readme once, so maybe this shouldn't be default behavior.