tests: Manually assign the tests distribution

[jlduran]

Summary

These Makefiles do not include bsd.test.mk because they do not contain
tests (i.e., they do not need a Kyuafile). They should be however, part
of the test distribution.

Manually assign the distribution, otherwise these files will get
assigned to the default base distribution (base.txz instead of
tests.txz).

Test Plan

This is part of a larger effort to reconcile freebsd-base(7) packages
with distributions (legacy).

In preparation for NanoBSD Reimagined (GSoC 2026).

The test plan is to compare the mtree output of both images, one built
using just the FreeBSD-set-base package and the other one built using
just the base.txz distribution sets.

[jlduran]

Hi @senguptaangshuman17_gmail.com, this revision is still missing a few tests. I plan to add them after this week's meeting.