lib/libc/tests/string: add memrchr unit tests

The "values" test case is specifically crafted to detect the off-by-one
error previous discovered in the scalar strchrnul implementation.

Tested by: developers@, exp-run
Approved by: mjg
MFC after: 1 month
MFC to: stable/14
PR: 275785
Differential Revision: https://reviews.freebsd.org/D42925