Determine "rpz" system test mode based on $mode rather than $DNSRPS_TEST_MODE

Determine "rpz" system test mode based on $mode rather than $DNSRPS_TEST_MODE

Change 4897 modified the way the $DNSRPS_TEST_MODE variable is used in
bin/tests/system/rpz/tests.sh without updating all references to it,
which i.a. causes the $native and $dnsrps variables to not be set in the
default testing mode, effectively preventing failed checks from being
propagated to the final result of the test.  Use $mode instead of
$DNSRPS_TEST_MODE where appropriate to fix error handling in the "rpz"
system test.