[gdb/testsuite] Fix gdb.python/python.exp with --disable-tui

When building gdb with --disable-tui, we run into:
...
(gdb) python print(type(gdb.TuiWindow))^M
Python Exception <class 'AttributeError'>: \
  module 'gdb' has no attribute 'TuiWindow'^M
Error occurred in Python: module 'gdb' has no attribute 'TuiWindow'^M
(gdb) FAIL: gdb.python/python.exp: gdb.TuiWindow is registered
...

Fix this by skipping the test for --disable-tui.

Tested on x86_64-linux.