fixml cannot evaluate repositories with no test folder.

[WeilinHan8]

There are some github repos that do have test files but not inside test folder (no test folder in the repository)(ie. https://github.com/ml4bio/USPNet) . In the case, fixml throws following error message:
IndexError: list index out of range

I suggest to add a test in fixml to handle this issue by:

1. Giving a more intuitive and informative error message when there is not test file to evaluate.
2. Changing the algorithm of fixml to search for test files (with "test" in the file name) instead of searching for files inside of test folder.