File URI's without a host are not UNC paths on Windows.

NUnit breaks when these URI's are treated as UNC paths, so I have no way of
running the tests on Windows without this change to see if it broke anything.
Not all URI tests pass, but the ones that fail seem unrelated to my change.

On Unix, the tests all work, but of course on Unix this is a no-op.

I have not been able to run tests in a Windows build of Mono since https://github.com/mono/mono/pull/1168