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

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

When running the test-case gdb.tui/wrap-line.exp with a build configured with
--disable-tui, we run into:
...
(gdb) PASS: gdb.tui/wrap-line.exp: width-hard-coded: set width 50
tui new-layout command-layout cmd 1^M
Undefined command: "tui".  Try "help".^M
(gdb) ERROR: Undefined command "tui new-layout command-layout cmd 1".
...

Fix this by guarding the command with allow_tui_tests.

Tested on x86_64-linux.