Add some tests of the filename splitting of WX_FILENAME objects.
Interestingly, the result of GetFuillPath on an empty path is
likely wrong as it will imply an absolute path from /. But this is
probably not a used code path. Document the current behaviour as passing
the test.
This unit tests covers the code reported by Coverity 183884 and 183894
(improper use of negative). The find_last_of can return "npos" (-1), but
it's stored as a size_t. This is correct according the WX docs,
and the substr() method it is passed to can handle npos too.
John Beard