Clean up test task definition

This removes a layer of indirection, directly declaring the task,
instead of a task that defines a task when invoking.

lib/foreman_tasks/tasks/test.rake

@@ -1,14 +1,12 @@
 namespace :test do
-  task :foreman_tasks => 'db:test:prepare' do
-    test_task = Rake::TestTask.new('foreman_tasks_test_task') do |t|
-      t.libs << ['test', "#{ForemanTasks::Engine.root}/test"]
-      t.pattern = ForemanTasks::Engine.root.join('test/**/*_test.rb')
-      t.test_files = [Rails.root.join('test/unit/foreman/access_permissions_test.rb')]
-      t.verbose = true
-      t.warning = false
-    end
-
-    Rake::Task[test_task.name].invoke
+  desc 'Foreman Tasks plugin tests'
+  Rake::TestTask.new(foreman_tasks: 'db:test:prepare') do |t|
+    t.libs << Rails.root.join('test')
+    t.libs << ForemanTasks::Engine.root.join('test')
+    t.pattern = ForemanTasks::Engine.root.join('test', '**', '*_test.rb')
+    t.test_files = [Rails.root.join('test', 'unit', 'foreman', 'access_permissions_test.rb')]
+    t.verbose = true
+    t.warning = false
   end
 
   desc 'Alias for test:foreman_tasks'