galera SST tests have a debug part, but we don't want to limit them
to fulltest2 builder. So, add support for test files that
have a debug part:

* add maybe_debug.inc and maybe_debug.combinations
* 'debug' combination is run when debug is available
* 'release' combination is run otherwise
* test wraps debug parts in if($with_debug) { ... }
* and creates ,debug.rdiff for debug results