libc tests: add tests for link_addr(3) and link_ntoa(3)

for now, since link_addr() has no way to indicate an error, these are
only positive tests which check the outcome of valid inputs.

Reviewed by: ngie, des, adrian
Approved by: des (mentor)
Differential Revision: https://reviews.freebsd.org/D50062

(cherry picked from commit 757e973fb2112ea442aa8990d991f406d407b6f7)

link_addr_test: use <cstddef>, not <sys/stddef.h>

<cstddef> is the correct header; this fixes the GCC build.

while here, sort the headers.

Fixes: 757e973fb211 ("libc tests: add tests for link_addr(3) and link_ntoa(3)")
Reviewed by: des
Approved by: des (mentor)
Differential Revision: https://reviews.freebsd.org/D50189

(cherry picked from commit bbffdfef3bf04b2f89027cb20e2019fdc74c4b1b)