Add in-tree mspec and ruby/spec

* For easier modifications of ruby/spec by MRI developers.
* .gitignore: track changes under spec.
* spec/mspec, spec/rubyspec: add in-tree mspec and ruby/spec.
  These files can therefore be updated like any other file in MRI.
  Instructions are provided in spec/README.
  [Feature #13156] [ruby-core:79246]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@58595 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

spec/rubyspec/library/stringio/readbyte_spec.rb

@@ -0,0 +1,20 @@
+require File.expand_path('../../../spec_helper', __FILE__)
+require 'stringio'
+require File.expand_path('../shared/readchar', __FILE__)
+
+describe "StringIO#readbyte" do
+  it_behaves_like :stringio_readchar, :readbyte
+
+  it "reads the next 8-bit byte from self's current position" do
+    io = StringIO.new("example")
+
+    io.send(@method).should == 101
+
+    io.pos = 4
+    io.send(@method).should == 112
+  end
+end
+
+describe "StringIO#readbyte when self is not readable" do
+  it_behaves_like :stringio_readchar_not_readable, :readbyte
+end