[gdb/testsuite] Fix gdb.python/tui-window-factory.exp for remote host

When running gdb.python/tui-window.exp with host board
local-remote-host-notty and target board native-gdbserver, I get:
...
FAIL: gdb.python/tui-window-factory.exp: msg_2: \
  check test_window box (box check: ul corner is l, not +)
...

The problem is that the result of Term::prepare_for_tui is not checked.

Fix this by adding the missing check.

Tested on x86_64-linux.