[gdb/testsuite] Add missing -no-prompt-anchor in gdb.base/vfork-follow-parent.exp

[gdb/testsuite] Add missing -no-prompt-anchor in gdb.base/vfork-follow-parent.exp

When running test-case gdb.base/vfork-follow-parent.exp it passes fine, but
when running it with "taskset -c 0" I run into:
...
(gdb) inferior 1^M
[Switching to inferior 1 [process 26606] (vfork-follow-parent-exit)]^M
[Switching to thread 1.1 (process 26606)]^M
(gdb) Reading symbols from vfork-follow-parent-exit...^M
FAIL: $exp: exec_file=vfork-follow-parent-exit: target-non-stop=on: \
  non-stop=off: resolution_method=schedule-multiple: inferior 1 (timeout)
...

Fix this by using -no-prompt-anchor.

Tested on x86_64-linux.

PR testsuite/31166
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31166