fix test of r42101 [Feature #6626]

rl_delete_text removes characters in line_buffer, but it doesn't move rl_point.
Therefore it may cause invalid rl_point.

On following case, test_input_metachar causes test_insert_text failure.
(test_input_metachar_multibyte) is skipped because of locale)
http://c5664.rubyci.org/~chkbuild/ruby-trunk/log/20130723T133302Z.log.html.gz

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42146 b2dd03c8-39d4-4d8f-98ff-823fe69b080e