test_curses.rb: add sentinel

* test/test_curses.rb (TestCurses#run_curses): add sentinel to get rid
  of read pty output after the child process terminated, which causes
  EIO on Linux.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@40182 b2dd03c8-39d4-4d8f-98ff-823fe69b080e