Fix resolving relative windows network paths

PR #1825 had a small problem with some of the test cases on Windows where some of the new test data looked like a network path (i.e. something starting with `\\drive\`) and was therefore giving different results on Windows.

This PR expands the tests to explicitly cover various network path cases and also fixes a possible bug in the network path recognition code where a path starting with `\\\` was being considered a network path, where I think we should be looking for `\\` followed by a non-slash character.

I'm opening this as a PR just to get some confirmation that this new behavior for Windows network paths makes sense and that it will be okay to not accept `\\\` as a valid network path.