Special treatment for Noop mutations

Otherwise mutant would run all matched example groups for a noop when it
only makes sense to run the first match

lib/mutant/killer/rspec.rb

@@ -20,7 +20,12 @@ module Mutant
       def run
         mutation.insert
 
-        groups = example_groups
+        groups =
+          if mutation.is_a?(Mutation::Neutral::Noop)
+            [example_groups.first]
+          else
+            example_groups
+          end
 
         unless groups
           $stderr.puts("No rspec example groups found for: #{match_prefixes.join(', ')}")