test: fixtures module returning absolute paths causes test failures in Windows with 'x' or 'u' in path after delimiter

* **Version**: any with `fixtures` module
* **Platform**: Windows
* **Subsystem**: test

Refs: https://github.com/nodejs/node/issues/16023 (see https://github.com/nodejs/node/issues/16023#issuecomment-334885754)

cc @jasnell 