Fix broken multiple folder support in src_folders

This PR fixes a bug with support for multiple folders in `src_folders`, as mentioned here:

https://groups.google.com/forum/#!topic/nightwatchjs/7OUMOn5Acj8

Currently, `run()` in `runner/run.js` calls `runFiles` in a way that causes each entry in `src_folders` to run at the same time. 

This PR gathers up a single `fullpaths` list using `runFiles` first, and then calls `runTestModule` with the results.

All tests passing.