Before, there was a `log/buffer.go`, but that design is not general, and
it introduces a lot of irrelevant `Content() (string, error) ` and
`return "", fmt.Errorf("not supported")` .
And the old `log/buffer.go` is difficult to use, developers have to
write a lot of `Contains` and `Sleep` code.
The new `LogChecker` is designed to be a general approach to help to
assert some messages appearing or not appearing in logs.
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