+testKit Add fishForMessages allowing selective expecting of msgs

[ktoso]

Motivation:

We sometimes need to "fish for some message in a stream", i.e. only capture some messages that a probe receives.

This is most useful with the event stream on the cluster, e.g. "fish only for reachability changes"

Funnily enough the naming is adopted from Akka, not just our usual fishy puns :-)

Modifications:

• add fishForMessages and fishFor(WhatType)

Result:

• allows easy implementation of tests which need to only look as some cluster events (used a lot on cluster hardening work)

[yim-lee]

<3 Very handy!

[ktoso]

Yup! Thanks for review <3