library-tests gives cryptic error when no path is supplied

[quinnchr]

I was working on adding some tests to library/nearest and it took me an embarrassingly long time to figure out why I couldn't get them to pass, mainly due to this cryptic error message:

unknown location(0): fatal error in "test_nearest_response": std::out_of_range: vector::_M_range_check: __n (which is 0) >= this->size() (which is 0)

Which is of course because I wasn't supplying a path to the test data and get_args() was returning an empty vector. I think a check should be added to get_args with a more meaningful error message and the unit testing docs should be more informative about how to run the tests.