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/file.go

@@ -253,15 +253,6 @@ func (log *FileLogger) deleteOldLog() {
 	})
 }
 
-// Content returns the content accumulated in the content provider
-func (log *FileLogger) Content() (string, error) {
-	b, err := os.ReadFile(log.Filename)
-	if err != nil {
-		return "", err
-	}
-	return string(b), nil
-}
-
 // Flush flush file logger.
 // there are no buffering messages in file logger in memory.
 // flush file means sync file from disk.