misc/57861: ATF tests fail to complete because directory not empty

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

>Number:         57861
>Category:       misc
>Synopsis:       ATF tests fail to complete because directory not empty
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    misc-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Tue Jan 16 14:50:00 +0000 2024
>Originator:     Andreas Gustafsson
>Release:        NetBSD-current from October 2023 or newer
>Organization:
>Environment:
System: NetBSD
Architecture: i386
Machine: i386
>Description:

The ATF test runs on the TNF i386 testbed sometimes fail to run to
completion because the ATF test framework crashes running the
lib/librumphijack/t_tcpip:nfs_autoload test case, with the error
message:

    nfs_autoload: terminate called after throwing an instance of 'tools::system_error'
  what():  tools::fs::rmdir: Cannot remove directory: Directory not empty

This appears to be a recent regression, as the logs show no such errors
before October 2023.  The full set of recorded failures is:

  http://releng.netbsd.org/b5reports/i386/2023/2023.10.06.08.49.42/test.log
  http://releng.netbsd.org/b5reports/i386/2023/2023.10.08.21.52.34/test.log
  http://releng.netbsd.org/b5reports/i386/2023/2023.11.10.20.44.58/test.log
  http://releng.netbsd.org/b5reports/i386/2023/2023.11.23.20.40.08/test.log
  http://releng.netbsd.org/b5reports/i386/2023/2023.12.13.10.53.15/test.log
  http://releng.netbsd.org/b5reports/i386/2023/2023.12.24.02.42.51/test.log
  http://releng.netbsd.org/b5reports/i386/2023/2023.12.27.17.35.37/test.log
  http://releng.netbsd.org/b5reports/i386/2024/2024.01.01.22.47.58/test.log
  http://releng.netbsd.org/b5reports/i386/2024/2024.01.06.17.32.41/test.log
  http://releng.netbsd.org/b5reports/i386/2024/2024.01.15.17.12.00/test.log

Similar failures have also occurred with the
net/if_vlan/t_vlan:vlan_l2tp test case on both i386 and amd64:

  http://releng.netbsd.org/b5reports/i386/2024/2024.01.13.23.59.47/test.log
  http://releng.netbsd.org/b5reports/i386/2024/2024.01.15.20.28.56/test.log
  http://releng.netbsd.org/b5reports/amd64/2024/2024.01.06.17.52.43/test.log

and with the net/if_vlan/t_vlan:vlan_l2tp test case on amd64:

  http://releng.netbsd.org/b5reports/amd64/2023/2023.12.10.20.17.23/test.log

>How-To-Repeat:

Run a testbed.

>Fix: