[gdb/testsuite] Add missing wait in gdb.python/tui-window-disabled.exp

While working on PR tui/30526, I noticed a bug in test-case
gdb.python/tui-window-disabled.exp.

Here we send "tui enable" to gdb, but don't wait for it to arrive before
checking for a window box:
...
    send_gdb "tui enable\n"
    Term::check_box "check for python window" 0 0 80 16
...

Fix this by waiting for the prompt to be issued in TUI before doing the check.

Tested on x86_64-linux.