lib/libc/tests/string/stpncpy_test.c: extend for upcoming SSE implementation

This adds additional unit tests validating the function for
All possible alignment offsets of source and destination.

Also extend the test to allow testing of an external stpncpy
implementation, which greatly simplifies the development of
custom implementations.

Sponsored by: The FreeBSD Foundation
Tested by: developers@, exp-run
Approved by: mjg
MFC after: 1 month
MFC to: stable/14
PR: 275785
Differential Revision: https://reviews.freebsd.org/D42519