Update expected results for tls13kexmodes tests

One of the scenarios constructed in these tests was erroneously
producing successful handshakes until the previous commits, but should
have been failing.  Update our expected behavior to match the
specification requirements, and adjust the commentary slightly for
a test case relevant for the other preceding commit.

Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/14749)

(cherry picked from commit 80c25611)