More parallel ktest

[WIP]

This turned out to be more involved then I expected, because ktest was designed for this. We knew that it's possible to improve concurrency of it but we decided to go with simpler approach at the time.

This is blocked by #1059 .

TODO:

[ ] KTest should check for required `-kompiled` directories when `--skip kompile` is provided, otherwise krun tests will fail. (this was working before this patch)
[ ] Posix-only steps are not implemented.