Skip the optional capi digest specs if fiddle is not installed

Should fix a failure on the OpenBSD RubyCI machine.

Author: jeremyevans

spec/ruby/optional/capi/digest_spec.rb

@@ -1,6 +1,10 @@
 require_relative 'spec_helper'
 
-require 'fiddle'
+begin
+  require 'fiddle'
+rescue LoadError
+  return
+end
 
 load_extension('digest')