Fix error in open-mpi tests; enable clang

It seems clang is more strict about stack corruption, so when it occurs it
makes the test fail when it doesn't on gcc. This fixes that corruption,
and all else works fine with clang.

Closes #292.

Signed-off-by: Xiyue Deng <manphiz@gmail.com>
open-mpi16: add reference to upstream ticket.