t/steadystate_tests: better support automated testing

t/steadystate_tests: better support automated testing

To better support using this script for automated testing, do the
following:

- Use a more portable python interpreter reference
- Drop six.moves dependency
- Eliminate two potential divide-by-zero runtime errors
- Count and report the number of passed and failed tests
- Keep the output of the test jobs for inspection in case of failure
- Exit code 0 if all tests pass and non-zero otherwise