lib/55331: libarchive test now fails randomly on real hardware

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

>Number:         55331
>Category:       lib
>Synopsis:       libarchive test now fails randomly on real hardware
>Confidential:   no
>Severity:       serious
>Priority:       high
>Responsible:    lib-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Sun May 31 20:25:00 +0000 2020
>Originator:     Andreas Gustafsson
>Release:        NetBSD-current
>Organization:
>Environment:
System: NetBSD
Architecture: x86_64
Machine: amd64
>Description:

The lib/libarchive/t_libarchive/libarchive test case recently started
failing randomly on real amd64 hardware, in about half the runs.  Logs
from the first failure recorded on my testbed are at:

  http://www.gson.org/netbsd/bugs/build/amd64-baremetal/commits-2020.05.html#2020.05.26.05.25.21

The failures happen in the "test_write_disk_sparse" part of the test.

>How-To-Repeat:

Run the ATF tests on real hardware, repeatedly.

>Fix: