tests: Rework tests/installed → tests/kola

Previously we made an effort to use the [Fedora Standard Test Interface](https://docs.fedoraproject.org/en-US/ci/standard-test-interface/).
This effort was not very successful; the primary thing that
it really died on is Ansible just didn't support rebooting
very well.  I think that's since gotten better, but even
then, Ansible wasn't the best thing for a test framework
for us anyways.

In the meantime Fedora CoreOS happened emphasizing Ignition
and not "post-hoc reconciliation" models like Ansible over
ssh.

And, [coreos-assembler](https://github.com/coreos/coreos-assembler) happened too.

Furthermore, we really need to test OSTree's interaction
with Ignition as we've invented several special things there.

Then most recently, I've been working on having
cosa/kola support running externally defined tests:
coreos/coreos-assembler#1215

There's a lot of things to clean up after this but at least this
works for me:

```
$ cd /srv/fcos
$ cosa kola run -- --parallel 4 --output-dir tmp/kola -E ~/src/github/ostreedev/ostree/ 'ext.ostree.*'
```

NOTE: This *does not* drop ostree binaries into the target.  See:
coreos/coreos-assembler#1252 (comment)

This drops our dependency on Python in the installed tests, and
also fixes a few bugs that came up.

I disabled the `itest-bare-user-root.sh` one because it's
entangled with the shell script infrastructure for the unit tests.