Check for bashism in system test

System test shell scripts are meant to be POSIX compliant but bashism constructs keep popping up (!4310 (merged), !988 (merged), !2548 (merged), !4235 (merged)).

There's a Debian checkbashisms tool which identifies such problems and we should investigate how reliable it is and then, potentially, integrate it to CI.

Or, with selected "codes", we can use shellcheck instead, e.g.: shellcheck -s sh -i SC2039 rpz/tests.sh.

Contrary to checkbashism, shellcheck is actively maintained.