Improve test logger (#24235)

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.

modules/log/provider.go

@@ -6,7 +6,6 @@ package log
 // LoggerProvider represents behaviors of a logger provider.
 type LoggerProvider interface {
 	Init(config string) error
-	Content() (string, error)
 	EventLogger
 }