Only load seeds once while testing

Since we use transactions now for every test, we can seed the database
at the beginning, and then it will go back to this state before a test
is executed.

Running the test suite is now considerably faster. On my machine, we
save a quarter of second per system test, meaning we save several
minutes for the whole suite.

spec/spec_helper.rb

@@ -20,11 +20,14 @@ RSpec.configure do |config|
   config.include(CommonActions)
   config.include(ActiveSupport::Testing::TimeHelpers)
 
+  config.before(:suite) do
+    Rails.application.load_seed
+  end
+
   config.before do |example|
     I18n.locale = :en
     Globalize.locale = nil
     Globalize.set_fallbacks_to_all_available_locales
-    load Rails.root.join("db", "seeds.rb").to_s
     Setting["feature.user.skip_verification"] = nil
   end