input verification - check if the folder doesn't already have ./work and ./results from tests

[golobor]

discovered by Joaquim Olle:
People who start working with distiller would run the test first, then modify the .yml file for their project and run it again. If the ./results and ./work folders are not deleted in between the two runs, distiller gets horribly confused and remaps the test data against the newly selected genome, which results in weird bugs.
We need to find a way to prevent this error. One easy solution is to store test results in ./test_results.