TestKit cleanups and making consistent

E.g. how to use expectMessage within eventually blocks?

Check if we are missing any overloads.

Decide what to do about ShouldMatchers -- internally we want them, but not as part of API or we're ok to expose them? Go we guarantee bin/source compat for the test utils?

Start by locating the TestKit entry points and existing uses of expectMessage within eventually blocks. Audit the available overloads and the ShouldMatchers exposure, then resolve the stated API and binary/source compatibility questions. Done means the cleanup scope and compatibility policy are agreed and the affected test utilities are consistent.

Written by the indexing model from the issue text.