ICMP checksum test: Fix for big endian

The in_cksum tests originally tried to simulate a BE environment by
swapping the byte order of the input. But that's overcomplicated, and
didn't actually work on real BE hardware. The correct testing strategy
is just to test on the native endianness, and run the tests in both BE
and LE environments.

Submitted by: Renato Riolino <renato.riolino@eldorado.org.br>
Reviewed By: asomers
Differential Revision: https://reviews.freebsd.org/D23193

(cherry picked from commit 292808246db702b9194deb8938e40fd06914aea9)