[RFC] Update run_tests.sh exit codes for "essential" tests, fixes #76

In this branch I updated the exit codes of `run_tests.sh` to the way it was before the introduction of essential tests, except that essential tests (fatal failures) are handled: You can see from the exit code, what the worst error that happened was (none, fatal failure, regular failure, broken tests, fixed tests).

#80 is related, as I moved the code that uses dc around.