[gdb/testsuite] Fix timeout in gdb.threads/inf-thr-count.exp

[gdb/testsuite] Fix timeout in gdb.threads/inf-thr-count.exp

With test-case gdb.threads/inf-thr-count.exp, check-readmore and
READMORE_SLEEP=1000 I run into:
...
(gdb) set variable spin = 0^M
(gdb) ^M
Thread 1 "inf-thr-count" hit Breakpoint 2, breakpt () at /data/vries/gdb/src/gdb/testsuite/gdb.threads/inf-thr-count.c:49^M
49      }^M
FAIL: gdb.threads/inf-thr-count.exp: set 'spin' flag to allow main thread to exit (timeout)
PASS: gdb.threads/inf-thr-count.exp: wait for main thread to stop
...

Fix this by using -no-prompt-anchor.

Tested on x86_64-linux.