Skip NFS TestShellBasedIdMapping tests that are irrelevant on Windows.

TestShellBasedIdMapping contains several tests that fail on Windows. The underlying implementation of ShellBasedIdMapping assumes a Unix-like environment, forking out to call things like bash and getent. The output parsing also assumes Unix-like output from those commands. The tests aren't currently relevant on Windows, because NFS isn't implemented for Windows. Whenever that work is done, ShellBasedIdMapping will need a much different implementation to support Windows.