Most of what is tested is already covered in the other 2 tests - ie.
lookup of slashed/non-slashed directory tasks, absence of comments on
"parent" tasks, and laziness of directory creation.
test_directory_win32 also looks at absolute paths (by prepending
Dir.pwd), but we don't have to worry about this case since it boils down
to the correctness about File.dirname; leave that to ruby core