Automatically expand scope of example groups

Given a subject Foo::Bar#baz mutant will run example groups with
description starting with:

  - Foo::Bar#baz
  - Foo::Bar
  - Foo

This basically means you can kill a mutation via a dedicated spec for a
given method, all specs for a given object or all the specs within the
object's namespace.

In most of the cases you should target killing mutations via a dedicated
spec for a particular method *however* there are valid cases where
implicit coverage is absolutely fine.

lib/mutant/killer/rspec.rb

@@ -29,7 +29,7 @@ module Mutant
 
         reporter = RSpec::Core::Reporter.new
 
-        example_groups.each do |group|
+        groups.each do |group|
           return true unless group.run(reporter)
         end
 
@@ -53,12 +53,7 @@ module Mutant
       # @api private
       #
       def example_groups
-        match_prefixes.each do |match_expression|
-          example_groups = find_with(match_expression)
-          return example_groups unless example_groups.empty?
-        end
-
-        nil
+        match_prefixes.flat_map { |prefix| find_with(prefix) }.compact.uniq
       end
 
       # Return example groups that match expression