make testing of ompi < v5 more robust

[garlick]

Problem: since PMI-1 is always active, it's not entirely clear that the testsuite covers using pmix to bootstrap ompi version < v5.

Disable PMI-1 service with -o mpi=off in the MPI tests. Add some checks in the mpi tests to ensure multiple singletons are not started.

Toss in another small change to zero more pmix_value_t objects in test programs.

[codecov[bot]]

Codecov Report

Merging #72 (2e7cad8) into main (9647b92) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##             main      #72   +/-   ##
=======================================
  Coverage   79.44%   79.44%           
=======================================
  Files          10       10           
  Lines        1231     1231           
=======================================
  Hits          978      978           
  Misses        253      253           

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