tests: Avoid some compiler warnings.

* tests/gpg/t-support.h (fail_with_syserr): New.
(check_data): Use it to avoid compiler warnings.
* tests/run-support.h (fail_with_syserr): New.

I don't think that there is a way to tell gcc that
gpgme_err_code_from_errno will never return 0. Thus we better use a
different macro.