Run unit tests on different systems

1. Check if jenkins instances can be registered as gitlab runners in order to expand existing gitlab jobs to more systems.
2. Convert the unittest_backend job to run on different systems, or if point 1 is not possible, add a jenkins job that runs backend unit tests on multiple systems.
3. Convert the unittest_ui job to run on different systems, or if point 1 is not possible, add a jenkins job that runs UI unit tests on multiple systems.

The systems used, should be all the supported versions of the most popular operating systems. At the time of writing, these are:

• 'alpine', '3.17', 'amd64'
• 'alpine', '3.18', 'amd64'
• 'alpine', '3.19', 'amd64'
• 'alpine', '3.19', 'aarch64'
• 'alpine', '3.20', 'amd64'
• 'alpine', '3.20', 'aarch64'
• 'rhel', '8', 'amd64'
• 'rhel', '9', 'amd64'
• 'rhel', '9', 'aarch64'
• 'debian', '10', 'amd64'
• 'debian', '11', 'amd64'
• 'debian', '11', 'aarch64'
• 'debian', '12', 'amd64'
• 'debian', '12', 'aarch64'
• 'fedora', '36', 'amd64'
• 'fedora', '39', 'amd64'
• 'fedora', '40', 'amd64'
• 'ubuntu', '20.04', 'amd64'
• 'ubuntu', '22.04', 'amd64'
• 'ubuntu', '22.04', 'aarch64'
• 'ubuntu', '24.04', 'amd64'
• 'ubuntu', '24.04', 'aarch64'

Apart from ensuring that unit tests pass on all systems, we also ensure that it builds on all systems, which might arguably be more useful.

We could also include building of the documentation in these jobs, which is currently problematic on some old Debian, Ubuntu, and RHEL systems, because of alabaster's version being too new and unsupported by the default python included in these systems.