Set --mpi slurm flag appropriately flag on some MPI tests

[msimberg]

This is essentially the same as #437, but applies the change to a few more tests. This also abstracts the setting of the appropriate flag into a mixin.

Tested with

UENV=prgenv-gnu/25.11:rc1,prgenv-gnu-openmpi/25.11:2132109439 \
reframe --config config/cscs.py -c checks/prgenv/helloworld.py \
-c checks/microbenchmarks/mpi/osu/osu_run.py \
-c checks/microbenchmarks/xccl/ \
--prgenv '(?!PrgEnv-ce)' --run

on daint. The latter uenv is in the build:: namespace from eth-cscs/alps-uenv#263. The same tests run on clariden, but prgenv-gnu-openmpi isn't deployed on santis, so has to be chosen directly using the squashfs path.

[msimberg]

cscs-ci run alps-daint-uenv;MY_UENV=prgenv-gnu/25.11:rc1,build::prgenv-gnu-openmpi/25.11:2132109439

[msimberg]

cscs-ci run alps-clariden-uenv;MY_UENV=prgenv-gnu/25.11:rc1

[msimberg]

cscs-ci run alps-daint-uenv;MY_UENV=prgenv-gnu/25.11:rc1

[msimberg]

NB. some of the osu tests sometimes fail the threshold even with this PR. This PR is only to get the tests actually running correctly to start with. Additionally, not all of the tests listed above are production tests. The hello world tests are, and were failing here for clariden: https://git.cscs.ch/alps/vcluster-validation/-/jobs/4210270.

[msimberg]

cscs-ci run alps-daint-uenv;MY_UENV=prgenv-gnu/25.11:rc1

[msimberg]

cscs-ci run alps-eiger-uenv;MY_UENV=prgenv-gnu/25.11:rc2

[msimberg]

I've moved the --mpi=pmix for a few CE test into a separate mixin. I don't want to put it into the generic ContainerEngineMixin because 1. not all containers necessarily need --mpi=pmix and with container we don't have a similar system of features as we have for uenvs, 2. many container engine tests don't use MPI so they don't need the flag at all, and 3. I'm unsure if some tests will misbehave if we set --mpi=pmix for all jobs. With all this, I've restricted my changes to the tests that I know relatively well and that are relevant for production.

For uenv we can safely set the --mpi flag because we know that they're using Cray MPICH if there's no feature, and if there is an openmpi/cray-mpich feature we can rely on that.

[msimberg]

Also note that on daint some of the CE tests are likely to fail because of the recent upgrade which broke PMIx support. I've run the tests on clariden successfully.

[msimberg]

cscs-ci run alps-daint-uenv;MY_UENV=prgenv-gnu/25.11:rc2

[msimberg]

cscs-ci run alps-eiger-uenv;MY_UENV=prgenv-gnu/25.11:rc2

[msimberg]

cscs-ci run alps-eiger-uenv;MY_UENV=prgenv-gnu/25.11:rc2

[msimberg]

cscs-ci run alps-santis-uenv;MY_UENV=prgenv-gnu/25.11:rc2