tests: tune compiler include paths

tests/Makefile.inc

tests/acl.c

tests/base32hex.c

tests/base64.c

tests/conf.c

tests/descriptor.c

tests/dname.c

tests/dnssec_keys.c

tests/dnssec_nsec3.c

tests/dnssec_sign.c

tests/dnssec_zone_nsec.c

tests/dthreads.c

tests/events.c

tests/fdset.c

tests/hattrie.c

tests/journal.c

tests/rrl.c

tests/rrset.c

tests/server.c

tests/slab.c