tests: fix checksum computation

This fixes an endianness bug in sys/netinet/ip_reass_test.
Just use the code from RFC 1071.

Reported by: glebius
Reviewed by: glebius, Timo Völker
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D57988

(cherry picked from commit fbc039e512c3bb1635ad20cc8f70ad608ea818b7)