Improve/refactor multistage tests

[dcermak]

There are multiple issues with the multistage test module:

• it does not forward the marks from containers and thus will always run all tests
• the amidst project is dead upstream and will eventually stop working (probably), we should find a replacement
• k3sup is not working on non-x86_64 and upstream is not willing to merge the 2 line fix: https://github.com/alexellis/k3sup/pull/345
• the adventureworks test is relying on the eol .Net 5

And we really are atm only testing openjdk 11, go on x86_64 and .Net 5.0. We should extend the test coverage here.

[dcermak]

An option for .Net would be the upstream containerization examples: https://github.com/dotnet/dotnet-docker

[dirkmueller]

see also #15