elide invalid tmp tests and update path testing to use filepath.FromSlash()

The `/tmp` and `/tmp/` tests were commented out because those tests are not valid in Windows. If the intent of these tests are to test temp directory paths, then they should be made os independent. If the intent of these two tests is something different, they should be replaced with tests that test the same things in a manner that will work on both Windows and *nix.

Added calls to `filepath.FromSlash()` to set the path separators to the OS specific value. This change resolves all path related test issues in `path_test.go`.

All remaining test failures are related to URLs, which must use `/` regardless of the host OS.