test_curses.rb: default TERM

* test/test_curses.rb (TestCurses#run_curses): default TERM
  environment variable which is needed by ncurses.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@40333 b2dd03c8-39d4-4d8f-98ff-823fe69b080e