[Test] Double the MPI ring job timeout to absorb cold cluster starts for stabilizing test_efa

[hehe7318]

Description of changes

The timeout handed to mpi_submit_openmpi.sh was 20 seconds on 48-slot instances, which left no headroom over the observed runtime. The first MPI launch in a freshly created cluster reads its binaries and libraries from an EBS root volume whose blocks are still lazy-loading from the AMI snapshot, inflating that one launch by an order of magnitude and making test_efa fail on grounds that have nothing to do with EFA.

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