Fixed warnings in tests.

src/dnslib/tests/dnslib/nsec3_tests.c

src/dnslib/tests/dnslib/packet_tests.c

src/dnslib/tests/dnslib/response2_tests.c

src/dnslib/tests/dnslib/zone_tests.c

src/dnslib/tests/realdata/dnslib/packet_tests_realdata.c

src/dnslib/tests/realdata/dnslib/response_tests_realdata.c

src/dnslib/tests/realdata/dnslib_tests_loader_realdata.c