lib/libc/tests/string: expand memcmp test to bcmp, timingsafe_{b,mem}cmp

The four functions more or less perform the same operation.
Reuse the same unit test with slight changes so we can cover
them all. Constant-time operation is not verified for the
timingsafe_* functions.

Sponsored by: The FreeBSD Foundation
Approved by: ngie
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D41528