Fix downgrade action

[JoshuaLampert]

In #109, I forgot to also run the tests manually instead of using julia-runtest as needed by the downgrade action when run with julia <1.12. Otherwise the tests do not use all downgraded package versions correctly. So this fixes this.

[JoshuaLampert]

Let me try one more thing quickly.

[JoshuaLampert]

It looks like we can also use Pkg.test instead of the rather ugly include("test/runtests.jl"). For some reason I previously thought that would use wrong package versions, but looking at https://github.com/DLR-AMR/T8code.jl/actions/runs/22585083955/job/65427604585?pr=110#step:7:36 it seems it uses the correct ones. Then I think Pkg.test is the more elegant solution, especially because we can keep the paths to files located in the test directory.

[JoshuaLampert]

Ahh, no, this indeed does not work. The package versions used in the tests are actually the ones starting here, which are partly the non-downgraded ones. So Pkg.test does re-resolve something and we need to manually do `include("test/runtests.jl)", which in turn means we need to adjust paths to files in the test directory. This is not ideal, but is be doable.

[JoshuaLampert]

Are you fine with changing the path to the files in the test directory like this, @ranocha? If yes, I think this is good to go.

[ranocha]

Thanks for all the work you put into fixing and updating downgrade actions!