Add colors to the output of the tests runner

Currently, the output of the test runner does not contain colors. But it should be possible add colors by changing the output of the [exercism reporter in elm-test-rs](https://github.com/mpizenberg/elm-test-runner/blob/2a6223fff9eba6cc568a1dccfdf722b0d461a652/src/ElmTestRunner/Reporter.elm#L161).