Use spec_helper for S/MIME settings specs

The constant tested is defined in an initializer, and these are not
loaded when using fast_spec_helper. When running just this test file, or
when running this test first this can lead to test failures. To fix this
we just use spec_helper instead of fast_spec_helper.

spec/config/smime_signature_settings_spec.rb

-require 'fast_spec_helper'
+require 'spec_helper'
 describe SmimeSignatureSettings do
   describe '.parse' do