Add warnings when only one of the test files exist

In some cases one can accidentally create a test file in the tests folder and one outside (yes i did make that mistake). Currently Kitty handles that by doing nothing, it would be cool if there was a warning along the lines of "The answer file for is missing". That would limit confusion a bit.

Whether or not it should be an option for Kitty to run the input file with no answer file is up for debate i guess.