[gdb/testsuite] Fix gdb.python/tui-window.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:
...
UNSUPPORTED: gdb.python/tui-window.exp: TUI not supported
FAIL: gdb.python/tui-window.exp: test title
...

Fix this by adding the missing return after the unsupported.

Tested on x86_64-linux.