Fix broken unit tests

lib/vagrant/action/builtin/mixin_synced_folders.rb

@@ -112,10 +112,12 @@ module Vagrant
                 raise "Internal error. Report this as a bug. Invalid: #{data[:type]}"
               end
 
-              if !impl_class[0].new.usable?(machine, true)
-                # Verify that explicitly defined shared folder types are
-                # actually usable.
-                raise Errors::SyncedFolderUnusable, type: data[:type].to_s
+              if !opts[:disable_usable_check]
+                if !impl_class[0].new.usable?(machine, true)
+                  # Verify that explicitly defined shared folder types are
+                  # actually usable.
+                  raise Errors::SyncedFolderUnusable, type: data[:type].to_s
+                end
               end
             end
 

plugins/providers/virtualbox/action/prepare_nfs_settings.rb

@@ -20,6 +20,7 @@ module VagrantPlugins
           opts = {
             cached: !!env[:synced_folders_cached],
             config: env[:synced_folders_config],
+            disable_usable_check: !!env[:test],
           }
           folders = synced_folders(env[:machine], **opts)
 

test/unit/plugins/providers/virtualbox/action/prepare_nfs_settings_test.rb

@@ -25,6 +25,10 @@ describe VagrantPlugins::ProviderVirtualBox::Action::PrepareNFSSettings do
 
   subject { described_class.new(app, env) }
 
+  before do
+    env[:test] = true
+  end
+
   it "calls the next action in the chain" do
     called = false
     app = lambda { |*args| called = true }