Turn status bar yellow while tests are running

Hello,

I was refactoring the code and the infinitest bar was remaining green all the time. Suddenly I understood that it was because I created an endless loop and the test has never finished. So I had to go back in the history to find the change introduced the endless loop. It was about 15 minutes ago.

In this pull request I have implemented the simplest way to let the user know when the test is running bu turning the status bar yellow.

I know that we could also consider some more subtle ways which would not make the bar change its color very frequently which could become uncomfortable. We could extend infinitest introducing a timeout so that it reports a failure or a warning after it is reached during the tests are running. If you have an idea what solution would be the best I would gladly implement it too.

Regards, Dimitry