[gdb/testsuite] Fix gdb.tui/main.exp without TUI

[gdb/testsuite] Fix gdb.tui/main.exp without TUI

With a build with --disable-tui, we get:
...
(gdb) PASS: gdb.tui/main.exp: set interactive-mode off
maint set tui-left-margin-verbose on^M
Undefined maintenance set command: "tui-left-margin-verbose on".  \
  Try "help maintenance set".^M
(gdb) FAIL: gdb.tui/main.exp: maint set tui-left-margin-verbose on
...

Fix this by adding the missing "require allow_tui_tests".

Tested on x86_64-linux.