feature: new block "---must_die"

Feature to test if exit value is something else than 0. Used to test
config parser can notice bogus config. If this is set, no requests
are done.

This is for #10.

I've update patch according to review from @agentzh
https://github.com/openresty/test-nginx/pull/11

In addition to previous version
- it is always failure to have coredump
- it is always failure not to be able to execute program
- supports repeat_each
- supports error_log
- handle situation where shell is used to run program