rpzrecurse test fails on v9_9_sub branch due to SERVFAIL cache

rpzrecurse test fails on v9_9_sub branch due to SERVFAIL cache interfering with the RCODE returned for clientip tests.

See: https://bind-build.isc.org//bind9/v9_9_sub/libtool/amd64-unknown-freebsd10.3/daemon2.lab.isc.org/default/2018-03-18_19:41:05_UTC/test.txt

I:rpzrecurse:starting resolver using named.clientip.conf
I:rpzrecurse:testing CLIENT-IP behavior #2 (63)
I:rpzrecurse:stopping resolver
I:rpzrecurse:starting resolver using named.clientip2.conf
I:rpzrecurse:test 63 failed: query failed
I:rpzrecurse:test 63 failed: query failed
I:rpzrecurse:test 63 failed: didn't get expected answer