Prepare the testing infrastructure for testing base image tarballs

[kanavin]

The actual tarballs tests are still being worked on, but in the process I've had to modify and refactor a few existing pieces. To make reviews more manageable, the changes are being offered in a pull request ahead of the actual tarball implementation.

[kanavin]

pytest is failing due to broken ubuntu mirror :-/

[t-8ch]

I applied the last three patches.

[t-8ch]

The elbevalidate package is meant for generic testing utilities, usable by everyone.
It is meant to be distributed as part of the Debian packages.

The functions here now don't fit this criteria. They are very much specific to the ELBE testing itself.

Also the testscript is inlined into the ELBE XML, for better traceability.
Moving the policy into the ELBE installation, is counter to this.

I am all for reusing the testing code. Maybe we can just use the same script for both tests and distinguish at runtime which image is being tested.
Or extend the XML to pass parameters to the script. (More effort)

[kanavin]

I guess the image name can be obtained from build_dir/source.xml with a pytest fixture, e.g. simple-validation-image has:

<project>
                <name>simple-validation-image</name>

Then the tests can parametrize themselves based on that value, or fail to run if the name isn't something they know. You ok with that?

[t-8ch]

That sounds good.