Add one version of "echo.rb" for each of stdout/stderr.

This makes is easier to write tests for both stdout_equal and stderr_equal.

t/bin/echo-stderr.rb

@@ -0,0 +1,3 @@
+#!/usr/bin/ruby
+
+STDERR.puts ARGV.join(" ")

t/bin/echo-stdout.rb

@@ -0,0 +1,3 @@
+#!/usr/bin/ruby
+
+puts ARGV.join(" ")