libc/tests/string: improve stpncpy() "bounds" unit test

The test is extended the same way I previously extended the memccpy()
test to fix what is probably the same kind of bug.

PR: 291359
Reported by: Collin Funk <collin.funk1@gmail.com>
Reviewed by: ngie
Approved by: markj (mentor)
Fixes: 6fa9e7d8737548ef93c573387ce62402c368d486 (D42519)
See also: 61ed5748e4e9c7397fcb2638b442f46ac5c9e7c5 (D46051)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D54169

lib/libc/tests/string/stpncpy_test.c: apply ngie's fixes

(cherry picked from commit 123c086200491819595abc271d360e605288fd18)
(cherry picked from commit 8be8642826099368208efb2c1222f2a102cc5b08)