Improvements to shell unit test runner

 - Now works in the FF24 shell in `utils/jsshell/js`
 - Takes paths relative to the current working dir instead of resolving paths relative to `test/`
 - Can process multiple test files. E.g., providing `test/unit/*` as an argument runs all tests in `test/unit`
Merge pull request #1312 from tschneidereit/nat

Improvements to shell unit test runner