more user variable tests


mysql-test/r/user_var.result:
  more user variable tests (just to have this behaviour written down somewhere)
mysql-test/t/user_var.test:
  more user variable tests (just to have this behaviour written down somewhere)
sql/sql_handler.cc:
  Bug#1826, HANDLER+ALTER TABLE=crash
  (unfortunately, it cannot be tested in mysql-test suite)