fail CI when successful tests leave stuff in TMPDIR

[davepacheco]

See #991. This approach won't work until #990 is fixed.

[davepacheco]

On commit d00eab3, the Ubuntu run fails as expected:
https://github.com/oxidecomputer/omicron/runs/6218199983?check_suite_focus=true#step:14:8

rmdir: failed to remove '/tmp/omicron_tmp': Directory not empty

and the saved artifact includes at least the expected files:

drwxr-xr-x   8 dap  staff     256 Apr 28 14:52 ./
drwx------@ 33 dap  staff    1056 Apr 28 14:52 ../
drwxr-xr-x@  6 dap  staff     192 Apr 28  2022 .tmp2TLg0X/
drwxr-xr-x@  6 dap  staff     192 Apr 28  2022 .tmp5AbIJ6/
drwxr-xr-x@  6 dap  staff     192 Apr 28  2022 .tmpwkn9vC/
drwxr-xr-x@  3 dap  staff      96 Apr 28  2022 internal-dns-test.MemxaLPsS91D/
drwxr-xr-x@  3 dap  staff      96 Apr 28  2022 internal-dns-test.V6irJIMKSeF8/

I'm not yet sure what's up with those .tmp directories. I didn't expect those here.

The buildomat log shows similar files and then fails as expected:

6174 | 2022-04-28T21:09:40.123Z | + echo 'files in /var/tmp/omicron_tmp (none expected on success):'
-- | -- | --
6175 | 2022-04-28T21:09:40.129Z | files in /var/tmp/omicron_tmp (none expected on success):
6176 | 2022-04-28T21:09:40.135Z | + find /var/tmp/omicron_tmp -ls
6177 | 2022-04-28T21:09:40.141Z | 197333   17 drwxr-xr-x   7 build    build           7 Apr 28 21:09 /var/tmp/omicron_tmp
...
6302 | 2022-04-28T21:09:40.927Z | 201530    1 -rw-r-----   1 build    build          15 Apr 28 21:06 /var/tmp/omicron_tmp/.tmpyYVTg9/data/cockroach.advertise-addr
-- | -- | --
6303 | 2022-04-28T21:09:40.933Z | 201513    2 -rw-r--r--   1 build    build         710 Apr 28 21:06 /var/tmp/omicron_tmp/.tmpyYVTg9/cockroachdb_stderr
6304 | 2022-04-28T21:09:40.938Z | 201279    1 -rw-r-----   1 build    build          50 Apr 28 21:06 /var/tmp/omicron_tmp/.tmpyYVTg9/listen-url
6305 | 2022-04-28T21:09:40.945Z | + rmdir /var/tmp/omicron_tmp
6306 | 2022-04-28T21:09:40.951Z | rmdir: directory "/var/tmp/omicron_tmp": Directory not empty
6307 | 2022-04-28T21:09:40.958Z | process exited: duration 2484565 ms, exit code 2

Next I'm going to resync with "main", which includes the fix for #990.

[davepacheco]

I found the cause of the three extra .tmp* directories by running groups of tests individually until I found the ones that were leaking stuff. (Yeah, this kind of sucks. It'd be better if we created these directories named by the test in question.) Anyway, I'm rolling that fix into this change. Between that and pulling in #993 I'm hopeful the next run will be clean.