libc: fix access mode tests in fmemopen(3)

Previously a stream opened as read-only could be written to. Add a test
case for the fix.

Also correct another incorrect access mode check that worked by
accident, and improve the tests for that.

PR: 281953
Reported by: Erkki Moorits, fuz
Reviewed by: fuz, khng (earlier)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D47265