Update tests

test/unit/vagrant/environment_test.rb

@@ -727,6 +727,16 @@ VF
         expect(subject.default_provider).to eq(:baz)
       end
     end
+
+    it "is VirtualBox if nothing else is usable" do
+      plugin_providers[:foo] = [provider_usable_class(false), { priority: 5 }]
+      plugin_providers[:bar] = [provider_usable_class(false), { priority: 5 }]
+      plugin_providers[:baz] = [provider_usable_class(false), { priority: 5 }]
+
+      with_temp_env("VAGRANT_DEFAULT_PROVIDER" => nil) do
+        expect(subject.default_provider).to eq(:virtualbox)
+      end
+    end
   end
 
   describe "local data path" do