Fix URITests to work under Windows

These 3 tests are broken under Windows, because they use paths starting with `"/"` and expect them to be absolute, whereas that is only the case under Unixes. With this fix, these 3 tests now pass under Windows as well:
```
[info] + URI Tests.directoryURI adds trailing slash: OK, proved property.
[info] + URI Tests.directoryURI preserves trailing slash: OK, proved property.
[info] + URI Tests.filePathURI and File.toURI agree for absolute file: OK, proved property.
```