fix regressiontests to correctly rerun and report the mdrun usage
Some recent changes improved the structure of how we customize our running of mdrun during tests, but as Roland noted at https://gerrit.gromacs.org/#/c/3186/1/gmxtest.pl and https://gerrit.gromacs.org/#/c/3223/ we sometimes re-run mdrun when we should report failure, or vice-versa.
Re-runs should occur in response to an identified error message, and the change to the mdrun command line should reflect that error. Unable to successfully run mdrun should result in an error condition reported correctly to the harness.
Fix backup and exit code issues
- Set GMX_MAXBACKUP to
1 (if not set by user) so that tests Check the exit value of grompp
don't fail after being run 99 times
- Don't rerun mdrun if mdrun failed with any error message
other than those explicitly handeled by how_should_we_rerun_mdrun
for parallelization reasons