fix(runner): Karma hangs when file paths have \u in them #924

Escape backslashes in paths, which happen in Windows,
to prevent bad IDs (e.g. aaa\uuuu).
Add unit test with path with backslashes.

Closes #924
Merge pull request #932 from MrP/master

fix(runner): Karma hangs when file paths have \u in them #924