quic-interop: exlude multiplexing

quic-interop: exlude multiplexing

The multiplexing test works on local runs, but
appears to be failing in CI, possibly due to some environmental
limitation (the test generates a large list of requests in an
environment variable), leading to not sending all the requests needed.
Disable the test for now, and look to re-enable it after release when we
can appropriately diagnose the problem

Reviewed-by: Viktor Dukhovni <viktor@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/25528)