Regression unit test for frozen string literal fix (#1393)

Add a unit test for the prepare nfs settings action to act as a
regression test for the recent fix to avoiding modifying a frozen string
literal.

As part of this fix how the communicator is returned via a call to a
machine for testing purposes and remove an obsolete expect from the
wait till up tests as the code that would result in the communicator
being called has been removed.

Ensure that nfsd is not required to run the tests by mocking out the
host capability check.

lib/vagrant-libvirt/util/nfs.rb

@@ -1,5 +1,7 @@
 # frozen_string_literal: true
 
+require 'vagrant/action/builtin/mixin_synced_folders'
+
 module VagrantPlugins
   module ProviderLibvirt
     module Util