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/smtp.go

@@ -95,11 +95,6 @@ func (log *SMTPLogger) sendMail(p []byte) (int, error) {
 	)
 }
 
-// Content returns the content accumulated in the content provider
-func (log *SMTPLogger) Content() (string, error) {
-	return "", fmt.Errorf("not supported")
-}
-
 // Flush when log should be flushed
 func (log *SMTPLogger) Flush() {
 }