run-tests: fix whitelist/blacklist with directories on Windows The file name is resolved with `os.path.relpath()` in the `Test` constructor, which yields `\` on Windows. That doesn't match the `/` separator when using MSYS tools to build the list, and it isn't obvious that this is the problem because directory separators can mostly be used interchangeably. The `--test-list` argument already seems to be properly handled.