strfmon_test: Add a test for strfmon_l

Attempt to test the correctness of strfmon_l(3).

Items marked with XXX represent an invalid output.

Obtained from: https://github.com/NetBSD/src/commit/e7eba0044fe6128291cbb7e5923c7cf7d87318cc

Reviewed by: kib
PR: 267410
Github PR: #620
MFC after: 1 week