Fix for run-tests.sh where it would fail if nosetests is missing yanc plugin

This fixes a case where run-tests.sh would abort when nosetests doesn't know about the yanc plugin (which is the case e.g in the standard debian nosetests package).
```
Running tests...
Usage: nosetests [options]

nosetests: error: no such option: --with-yanc

FAILURE
```
Instead it will notify about the missing color output and continue with this patch.