misc/54451: net/ndp/t_ndp:ndp_cache_expiration test case fails randomly

[gson%gson.org@localhost (Andreas Gustafsson)]

>Number:         54451
>Category:       misc
>Synopsis:       net/ndp/t_ndp:ndp_cache_expiration test case fails randomly
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    misc-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Sat Aug 10 14:15:00 +0000 2019
>Originator:     Andreas Gustafsson
>Release:        NetBSD-current
>Organization:

>Environment:
System: NetBSD guava.gson.org
Architecture: x86_64 and i386
Machine: amd64 and i386
>Description:

The ndp_cache_expiration test case of the net/ndp/t_ndp test program
is failing randomly both under qemu and on real hardware.  Here are
links to a recent failure from each:

  http://releng.netbsd.org/b5reports/i386/2019/2019.08.09.17.22.02/test.html#net_ndp_t_ndp_ndp_cache_expiration
  http://www.gson.org/netbsd/bugs/build/amd64-baremetal/2019/2019.08.10.08.46.09/test.html#net_ndp_t_ndp_ndp_cache_expiration

The first failure on b5 was shortly after this commit:

  2019.07.18.04.00.09 ozaki-r src/tests/net/ndp/t_ndp.sh 1.33

with the commit message "tests: shorten the expire time of neighbor
caches to reduce the runtime of the tests".

>How-To-Repeat:

Run the ATF tests repeatedly.

>Fix: