Update path_to_executable spec so it doesn't simply mirror implementation.

- Verify the file exists.
- Verify it's an executable.

Author: myronmarston

lib/rspec/core.rb

@@ -139,7 +139,7 @@ class << self
 
     # @private path to executable file
     def self.path_to_executable
-      File.expand_path('../../../exe/rspec', __FILE__)
+      @path_to_executable ||= File.expand_path('../../../exe/rspec', __FILE__)
     end
 
   end

spec/rspec/core_spec.rb

@@ -55,7 +55,9 @@
 
   describe "::Core.path_to_executable" do
     it 'returns the absolute location of the exe/rspec file' do
-      expect(RSpec::Core.path_to_executable).to eq File.expand_path('../../../exe/rspec',__FILE__)
+      expect(File.exist? RSpec::Core.path_to_executable).to be_true
+      expect(File.executable? RSpec::Core.path_to_executable).to be_true
     end
   end
 end
+