[gdb/testsuite] Fix gdb.threads/clone-attach-detach.exp

[gdb/testsuite] Fix gdb.threads/clone-attach-detach.exp

With test-case gdb.threads/clone-attach-detach.exp I usually get:
...
(gdb) attach <pid> &^M
Attaching to program: clone-attach-detach, process <pid>^M
[New LWP <lwp>]^M
(gdb) PASS: $exp: bg attach <n>: attach
[Thread debugging using libthread_db enabled]^M
Using host libthread_db library "/lib64/libthread_db.so.1".^M
...
but sometimes I run into:
...
(gdb) attach <pid> &^M
Attaching to program: clone-attach-detach, process <pid>^M
[New LWP <lwp>]^M
(gdb) [Thread debugging using libthread_db enabled]^M
Using host libthread_db library "/lib64/libthread_db.so.1".^M
FAIL: $exp: bg attach <n>: attach (timeout)
...

I managed to reproduce this using make target check-readmore and
READMORE_SLEEP=100.

Fix this using -no-prompt-anchor.

Tested on x86_64-linux.

Approved-By: Simon Marchi <simon.marchi@efficios.com>