getRelativePath uses OS specific path separators while Karma always uses UNIX ones

[f1ames]

The troublesome line is

karma-ckeditor4-preprocessor/index.js

Line 25 in bee66ae

start = `${ path.sep }${ start }${ path.sep }`;

While run on windows path.sep is \, but Karma still uses / in file paths which causes the above method not to work properly.