Add test for rdbg executable

Author: larskanis

test/test_executables.rb

@@ -36,6 +36,10 @@ def test_rake
     assert_match(/rake, version/, `rake --version`)
   end
 
+  def test_rdbg
+    assert_match(/rdbg /, `rdbg --version`)
+  end
+
   def test_rdoc
     assert_match(/\d+\.\d+\.\d+/, `rdoc --version`)
   end