Refactor specs to stop using alternate config/world instances.

The use of alternate world and config instances was getting
in the way of some changes I want to make related to group
registration and isn't needed since we sandbox things so each
spec runs with fresh world and config instances.

Author: myronmarston

spec/rspec/core/configuration_spec.rb

@@ -21,19 +21,18 @@ module RSpec::Core
     end
 
     describe '#on_example_group_definition' do
-      let(:configuration) do
-        tmp_config = RSpec::Core::Configuration.new
-        tmp_config.on_example_group_definition do |example_group|
-          example_group.examples.first.metadata[:new_key] = :new_value
+      before do
+        RSpec.configure do |c|
+          c.on_example_group_definition do |example_group|
+            example_group.examples.first.metadata[:new_key] = :new_value
+          end
         end
-        tmp_config
       end
-      let(:world) { RSpec::Core::World.new(configuration) }
 
       it 'successfully invokes the block' do
         example_group = RSpec.describe("group") { it "example 1" do; end}
-        world.register(example_group)
-        example = world.example_groups.first.examples.first
+        RSpec.world.register(example_group)
+        example = RSpec.world.example_groups.first.examples.first
         expect(example.metadata[:new_key]).to eq(:new_value)
       end
     end

spec/rspec/core/example_group_spec.rb

@@ -359,11 +359,10 @@ def ascending_numbers
       end
 
       it "is not registered in world" do
-        world = RSpec::Core::World.new
         parent = RSpec.describe
-        world.register(parent)
         parent.describe
-        expect(world.example_groups).to eq([parent])
+
+        expect(RSpec.world.example_groups).to eq([parent])
       end
     end
 

spec/rspec/core/world_spec.rb

@@ -4,8 +4,8 @@ class Foo; end
 module RSpec::Core
 
   RSpec.describe RSpec::Core::World do
-    let(:configuration) { RSpec::Core::Configuration.new }
-    let(:world) { RSpec::Core::World.new(configuration) }
+    let(:configuration) { RSpec.configuration }
+    let(:world) { RSpec.world }
 
     describe '#reset' do
       it 'clears #example_groups' do