Make log/logtest more convenient

[pellared]

Issues discovered so far:

1. It is not easy to compare a whole recording done by Recorder.
2. Each new Logger the same name created via Recorder produces a new ScopeRecords element. This is not following the Logs model as the records produced by these loggers are within the same scope.
3. AssertRecordEqual is the only helper which is also not convenient as creating a log.Record is not straightforward.
4. Testing context.Context passed to Emit is not straightforward (not handled by AssertRecordEqual).
5. RecordFactory is unused.

Prototype: #6464