Change the concept of gtest::CoreLog so it becomes a log of the user related event

[techraed]

Proposal

Currently gtest::CoreLog and gtest::ExtManager.log is about all the user related messages either they are sent to mailbox or not. That must be changed, so log becomes collection of the user related events, emitted when:

1. user gasful message with zero gas sent
2. delayed user gasful message with zero gas sent
3. any kind of replies to user are sent

To test that some user message was added to the mailbox a provided in gtest mailbox API must be called.

[techraed]

done in #4232