fix tests on *NIX (skip a test, use /tmp instead of C:\..)

[fohrloop]

Replaces: https://github.com/fohrloop/dash-uploader/pull/136

• The tests were failing on linux/macOS, since one of the tests was testing a case where a file could not be used since it was used on another process. That problem was originally only seen on Windows, and so far it seems that it's not possible to reproduce on linux
• Also using /tmp/dash-uploader-uploads instead of C:\tmp\Uploads for the test data upload location on *nix.