Tests that fail when using @Grab with Extension Modules

See [GROOVY-7225](http://jira.codehaus.org/browse/GROOVY-7225)

- Adds .DS_Store as an exclusion via .gitignore
- Fixes an issue with `ExtensionModuleHelperForTests` that ignored test failures (try the existing tests with `assert false` and the tests still pass)
    - Two of the three existing tests now fail on both:
        - Groovy Version: 2.3.9 JVM: 1.8.0_25 Vendor: Oracle Corporation OS: Mac OS X
        - Groovy Version 2.3.9 JVM: 1.7.0_65 Vendor: Oracle Corporation OS: Linux
- Adds a new test showing failure with extension modules that add methods to `Closure`s