This introduces some code in tests for performing live tests of the

installer across all supported Linux distributions.
A docker-compose.yaml is responsible for launching some Docker

infrastructure for package caching, which is strongly encouraged for

tests. A rbinstall-test.ini should then be generated from

rbinstall-test.ini.tmpl to specify the cache addresses, and optionally

RHEL license information.
run-install-tests.py can then be used to perform the tests. This will,

by default, run through every supported Linux distribution sequentially.

It can instead run subsets, and can run tests in parallel.
Log information will be output to logs/, to aid debugging.

All tests pass on all supported Linux distributions.