Fix path generation on non-posix systems

[spectras]

This is the issue I talked about here.

Using path.join generates a path that uses platform's separator. But it seems karma uses posix style everywhere, so path should explicitly use posix, otherwise it breaks on Windows.

I do not really know how to make a unit test for this.

For reference, without this fix, 7.0.4 gives this kind of output on Windows:

INFO [preprocessor.rollup]: Generating bundle for ./tests\index.js
INFO [karma-server]: Karma v4.4.1 server started at http://localhost:9876/
INFO [launcher]: Launching browsers ChromeHeadless with concurrency unlimited
INFO [launcher]: Starting browser ChromeHeadless
INFO [HeadlessChrome 80.0.3987 (Windows 10.0.0)]: Connected on socket x70xvyh_mjMyqZ1LAAAA with id 31138964
WARN [web-server]: 404: /absoluteD:/projects/skeleton/skeleton-lib/tests/index.js?bee65b484288e8bc11a0b43f3c9778762b209801
WARN [web-server]: 404: /absoluteD:/projects/skeleton/skeleton-lib/tests/index.js?bee65b484288e8bc11a0b43f3c9778762b209801
HeadlessChrome 80.0.3987 (Windows 10.0.0): Executed 0 of 0 SUCCESS (0.002 secs / 0 secs)

So no error, but 0 test running ^^

[spectras]

Tested 7.0.5. Works fine, including watching.
Thanks for being so fast at checking issues :)