tests: fixed arguments processing in run-tests.sh

The 'seq' is a command, not a shell internal, and can be missing (e.g. Cygwin).

And old behaviour was wrong. The 'shift' in '-c' and '-j' blocks should move the $i counter. Otherwise some argumens can change the position.

Note: There exists shorter $i-- expressions, but compatibility won.
Merge pull request #1210 from milo/fix-run-tests

Fixed arguments processing in run-tests.sh