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.