tests/multi-server: put kafka-produce back in the CI short suite

The client-cache fix in b56deab ("Create different client cache
lists for TCP and UDP") addresses the underlying reason the
kafka-produce test was timing out in CI - the load-generator's own
Status-Server client lookup was failing the same way as
proxy-multihop-accept's.  Rename short.test.yml back to
short.ci.test.yml so it runs under test.multi-server.ci again.

Author: arr2036