Add haml-spec to tests after other tests

See #656.

If `haml-spec/ruby_haml_test.rb` is the first test file to be loaded by
Rake, Haml is loaded without Rails already being loaded. This means
`haml/railtie` isn't loaded which causes some test failures that expect
it to have been.

Explicitly add `haml-spec/ruby_haml_test.rb` after the "normal" test
files so this doesn't happen.

Rakefile

@@ -20,7 +20,11 @@ end
 
 Rake::TestTask.new do |t|
   t.libs << 'lib' << 'test'
-  t.test_files = Dir["test/**/*_test.rb"]
+  # haml-spec tests are explicitly added after other tests so they don't
+  # interfere with the Haml loading process which can cause test failures
+  files = Dir["test/*_test.rb"]
+  files.concat(Dir['test/haml-spec/*_test.rb'])
+  t.test_files = files
   t.warning = true
   t.verbose = true
 end