awk: Move to using two sets of tests

Upstream one-true-awk has two sets of tests. These are in addition to
NetBSD's tests we're using. The 'bugs-fixed' tests from upstream are
ready to use as-is (more or less). However, the 'tests' from upstream
are not, so for now we'll just use the netbsd and bugs-fixed tests.
They provide an OK workout and are better than nothing, though the tests
themselves are for specific esoteric things.

The upstream bugs-fixed tests are *ALMOST* a drop in. However, 3 test
for errors and the upstream test jig mashes stdout and stderr together,
which atf doesn't do, so make a tiny tweak to the upstream tests that I
hope to upstream. Plus upstream has ../a.out: instead of awk: in the
output. Not sure how to deal with this yet, so I've not proposed
anything upstream and have changed the test locally.

In addition, the system-status.awk test is not suitable to run in ATF.
It wants to force sh to dump core, but kyua doesn't seem to allow that
sometimes so the test will fail or pass based on whether or not a core
dump can be created. Since it's unstable, remove it.

This required moving the netbsd tests to a new direcotry, so update
mtree files as well. The change is useless for 'make check' without it.

Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D31376