[gdb/testsuite] Fix timeout in gdb.multi/attach-while-running.exp

[gdb/testsuite] Fix timeout in gdb.multi/attach-while-running.exp

With test-case gdb.multi/attach-while-running.exp usually I get:
...
(gdb) run &^M
Starting program: attach-while-running ^M
(gdb) PASS: $exp: run &
[Thread debugging using libthread_db enabled]^M
Using host libthread_db library "/lib64/libthread_db.so.1".^M
add-inferior^M
[New inferior 2]^M
Added inferior 2 on connection 1 (native)^M
(gdb) PASS: $exp: add-inferior
...
or:
...
(gdb) run &
Starting program: attach-while-running
(gdb) PASS: $exp: run &
add-inferior
[New inferior 2]
Added inferior 2 on connection 1 (native)
(gdb) PASS: $exp: add-inferior
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib64/libthread_db.so.1".
...
but sometimes I run into:
...
(gdb) run &
Starting program: attach-while-running
(gdb) PASS: $exp: run &
add-inferior
[New inferior 2]
Added inferior 2 on connection 1 (native)
(gdb) [Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib64/libthread_db.so.1".
FAIL: $exp: add-inferior (timeout)
...

Fix this by using -no-prompt-anchor.

Tested on x86_64-linux.