Use sc_venv_template for tests on JUWELS

[brownbaerchen]

second try.. This time without git submodules.

[brownbaerchen]

In this PR, I changed the setup of the virtual environments that are used in tests on JUWELS within the CI pipeline.
Specifically, I adapted the sc_venv_template by Stefan Kesselheim.

Conveniently, this can be used outside of the environment as well. I have a couple of these environments for different machines in the GPU project and we can consider adding more of these to the accessible pySDC/etc/ location for people to get started quickly on Jülich machines.

This also allows to install the venv in your account, run the tests until they pass, and then be reasonably confident that the pipeline is using the same environment.

Furthermore, this now uses the modules from the 2025 stage, which had caused trouble in #544 for reasons we still don't understand.

Note that this venv is installed from scratch every time. I previously hinted at the option to not do that. However, a fresh install may be a good idea and it doesn't take that long.

Supersedes #544
Closes #574

[tlunet]

Nice, thanks for working on this 😍

[jakob-fritz]

I like the restructuring. However, I found one line (in multiple occasions) not really easy to understand and would have appreciated a comment on what it does.
This is marked.

Furthermore, I don't know the reason for the change in test_spectral_helper.py (also marked with a comment), but assume you know what you did there and why you did it.

[jakob-fritz]

Thank you for the additions. Looks good to me now