"dnssec" system test: add "-r $RANDFILE" where it is missing

If the path to the source of random data is not passed explicitly to dnssec-keygen and dnssec-signzone and the --with-randomdev compile-time switch is not used, the aforementioned utilities will hang if the default source of random data (/dev/random) runs out of entropy. Use "-r $RANDFILE" to prevent that from happening during the setup stage of the "dnssec" system test.