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

test/test_curses.rb

@@ -28,14 +28,16 @@ rescue Exception => e
 ensure
   close_screen
   puts "", [Marshal.dump([result, e])].pack('m').delete("\n")
+  print "\\0"
 end
 src
       if input
         w.print(input)
         w.flush
       end
-      res = r.read
+      res = r.gets("\0")
       return unless res
+      res.chomp!("\0")
       res, error = Marshal.load(res[/(.*)\Z/, 1].unpack('m')[0])
       raise error if error
       return res