Not a bug but to improve debug experience

While open `http://localhost:9876/debug.html` to debug the tests:

It seems that there isn't a way like
```
http://localhost:9876/debug.html?file=run-this-test-only.js
```
to execute several specified but not all the tests without changing `karma.conf.js` & restarting karma server.

Did i miss something?

And i make some changes on `karma/lib/middleware/karma.js` to meet the demand. i wonder whether it's in need.