fix Install test adios_c_test w/MPI
Created by: germasch
Not sure how that passed the CI before, maybe it got exposed by my PR that switches no-mpi mode per communicator.
Actually, that's probably the explanation, this passed the CI before my last update to the no-mpi unification thing, and the combination of merging those two then failed. But I'd argue it's clearly a bug in the test, fixed here.