argThat is not descriptive enough for composite conditions.

Long story:

As you may have noticed, the tests that use a single verify call use argThat to look at the payload. If a test passes this works well and if it fails it's straightforward to figure out where the issue is - there's only one call and because we use assertions inside argThat we'll know exactly what failed, instead of returning a boolean and causing a situation where argThat would just print the whole String for every call, which has bonus problems for us since we only really care about the calls that are done for the playback_session name, but there are more than those for the same method and all of them will be printed out.

But for tests with more than one PlayLog event verification this approach does not work: we cannot use assertions inside argThat because that will prevent verifications from passing when written in an order different to that in which the corresponding calls are verified, and since we cannot guaranteed that order, relying on it is a no-no. We could use argThat as intended instead of with assertions and just return a boolean though, that works - but if the test fails, it becomes really hard to figure out which call failed and why.

This PR introduces a mechanism to tackle this issue.