Testing Framework - Discrepancies in way stderr automatically combining with stdout in threaded versus MPI run

[feldergast]

In the testing framework call run_sst, stderr is automatically piped to stdout if no file is provided for stderr to be written. This works correctly for serial and threaded runs. However, in MPI runs, stderr is not automatically piped to stdout and stderr is lost if no file is specified for stderr to be written to. This is due to the way MPI handles stdout and stderr, which writes a file per rank for each of stdout and stderr. The framework then combines these individual files into a single file, but doesn't first combine stdout and stderr for the case where no stderr file is specified.