Tests that fail when using @Grab with Extension Modules

- Adds .DS_Store as an exclusion from .gitignore
- Fixes an issue with ExtensionModuleHelperForTests that ignored test failures (try the existing tests with `assert false` and the tests still pass)
- Adds test showing failure with extension modules that add methods to `Closure`s
GROOVY-7225: Fixed case where an extension module works on Closure