core: improve TestLogRebirth (#20961)

This is a resubmit of #20668 which rewrites the problematic test
without any additional goroutines. It also documents the test better.

The purpose of this test is checking whether log events are sent
correctly when importing blocks. The test was written at a time when
blockchain events were delivered asynchronously, making the check hard
to pull off. Now that core.BlockChain delivers events synchronously
during the call to InsertChain, the test can be simplified.

Co-authored-by: BurtonQin <bobbqqin@gmail.com>