feat: Rerun tests after change without waiting result of previous run

Currently, Karma [waiting until we finish execution of previous run before rerun tests on with new changes](https://github.com/karma-runner/karma/blob/master/lib/executor.js#L24). But this behavior doesn't work very well in `watch` mode with a lot of tests. It would be nice to have an option which allow don't wait finishing previous run.

https://github.com/karma-runner/karma/issues/2778

Start with lib/executor.js at the linked line and read issue #2778 for the related discussion. Trace how watch mode handles a new change while a run is active; done means a configurable option allows reruns without waiting for the previous run to finish.

Written by the indexing model from the issue text.