ci: Install kola tests

[cgwalters]

This builds on
coreos/coreos-assembler#1441
to install our tests rather than running them from the source
directory. This model will more cleanly allow us to ship
our tests along with a test container or elsewhere, separate
from the source directory.

Also prep for #2048

[jlebon]

One comment, but LGTM as is!

[cgwalters]

To expand on this, there's a few directions we could go with this.

One I am leaning towards a bit is where we have a quay.io/coreos/tests container that does:

FROM quay.io/coreos-assembler:latest
RUN git clone https://github.com/ostreedev/ostree && cd ostree && make -C tests/kola install
RUN git clone https://github.com/coreos/ignition && cd ignition && make -C tests/kola install

etc.

There's already an ostree-tests RPM...but using it would be a bit awkward anyways because for cosa we need to support src != target. Plus I'd like to support updating the tests without updating the content.

[dustymabe]

I don't think I have an opinion here. Thanks for continuing to keep testing at a high priority.

[jlebon]

/lgtm

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