Add test for provider priority fix

This commit adds tests for possible future regressions for the bug fixed
in the commit: "Fix Vagrant not prioritizing configured providers
correctly".

Two very similar tests were added because whether the bug manifests
or not depends on the order in which the provider dictionary keys
are iterated, which is specific to the dictionary implementation.

test/unit/vagrant/environment_test.rb

@@ -866,6 +866,30 @@ VF
       end
     end
 
+    it "is the provider in the Vagrantfile that is usable even if only one specified (1)" do
+      subject.vagrantfile.config.vm.provider "foo"
+      subject.vagrantfile.config.vm.finalize!
+
+      plugin_providers[:foo] = [provider_usable_class(true), { priority: 5 }]
+      plugin_providers[:bar] = [provider_usable_class(true), { priority: 7 }]
+
+      with_temp_env("VAGRANT_DEFAULT_PROVIDER" => nil) do
+        expect(subject.default_provider).to eq(:foo)
+      end
+    end
+
+    it "is the provider in the Vagrantfile that is usable even if only one specified (2)" do
+      subject.vagrantfile.config.vm.provider "bar"
+      subject.vagrantfile.config.vm.finalize!
+
+      plugin_providers[:foo] = [provider_usable_class(true), { priority: 7 }]
+      plugin_providers[:bar] = [provider_usable_class(true), { priority: 5 }]
+
+      with_temp_env("VAGRANT_DEFAULT_PROVIDER" => nil) do
+        expect(subject.default_provider).to eq(:bar)
+      end
+    end
+
     it "is the highest usable provider outside the Vagrantfile" do
       subject.vagrantfile.config.vm.provider "foo"
       subject.vagrantfile.config.vm.finalize!