libc/tests/string: add a more comprehensive unit test for strrchr()

The unit tests are patterned after those for memrchr().
This catches the issue found in 293915.

PR: 293915
Reviewed by: strajabot
Reported by: safonov.paul@gmail.com
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D56037