Simplify the various Snippet factories

spec/factories/personal_snippets.rb

 FactoryGirl.define do
   factory :personal_snippet, parent: :snippet, class: :PersonalSnippet do
-    trait :public do
-      visibility_level PersonalSnippet::PUBLIC
-    end
-
-    trait :internal do
-      visibility_level PersonalSnippet::INTERNAL
-    end
-
-    trait :private do
-      visibility_level PersonalSnippet::PRIVATE
-    end
   end
 end

spec/factories/project_snippets.rb

 FactoryGirl.define do
-  factory :project_snippet do
+  factory :project_snippet, parent: :snippet, class: :ProjectSnippet do
     project
-    author
-    title
-    content
-    file_name
   end
 end

spec/factories/snippets.rb

@@ -12,5 +12,17 @@ FactoryGirl.define do
     title
     content
     file_name
+
+    trait :public do
+      visibility_level Snippet::PUBLIC
+    end
+
+    trait :internal do
+      visibility_level Snippet::INTERNAL
+    end
+
+    trait :private do
+      visibility_level Snippet::PRIVATE
+    end
   end
 end