port-evbarm/59560: evbarm tmpfs /tmp is too small for ATF tests

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

>Number:         59560
>Category:       port-evbarm
>Synopsis:       evbarm tmpfs /tmp is too small for ATF tests
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    port-evbarm-maintainer
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Sun Jul 27 14:05:00 +0000 2025
>Originator:     Andreas Gustafsson
>Release:        NetBSD-current, source date >= 2024.12.27.19.02.19
>Organization:
>Environment:
System: NetBSD
Architecture: arm
Machine: evbarm
>Description:

The number of failing tests on the evbarm-earmv7hf testbed increased sharply
with this commit:

  2024.12.27.19.02.19 christos src/distrib/utils/embedded/conf/evbarm.conf 1.43
  2024.12.27.19.02.19 christos src/distrib/utils/embedded/conf/evbmips.conf 1.5
  2024.12.27.19.02.19 christos src/distrib/utils/embedded/conf/evbppc.conf 1.2
  2024.12.27.19.02.19 christos src/distrib/utils/embedded/conf/riscv.conf 1.3
  2024.12.27.19.02.19 christos src/distrib/utils/embedded/conf/x86.conf 1.12
  2024.12.27.19.02.19 christos src/distrib/utils/embedded/mkimage 1.85

as seen here:

  https://releng.netbsd.org/b5reports/evbarm-earmv7hf/commits-2024.12.html#2024.12.27.19.02.19

This commit had the effect of adding a tmpfs mount for /tmp which was
previously on the root file system.  The tmpfs is limited to 25% of
RAM and the testbed VM has 128 MB of RAM, so /tmp is now only 32 MB,
which is too little to reliably run the ATF tests.

See PR 54886 for some possibly relevant discussion about a similar issue
on i386.

>How-To-Repeat:

Run the ATF tests on an evbarm system with 128 MB of RAM.

>Fix: