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/event.go

@@ -247,12 +247,6 @@ func (m *MultiChannelledLog) GetEventLogger(name string) EventLogger {
 	return m.loggers[name]
 }
 
-// GetEventProvider returns a sub logger provider content from this MultiChannelledLog
-func (m *MultiChannelledLog) GetLoggerProviderContent(name string) (string, error) {
-	channelledLogger := m.GetEventLogger(name).(*ChannelledLog)
-	return channelledLogger.loggerProvider.Content()
-}
-
 // GetEventLoggerNames returns a list of names
 func (m *MultiChannelledLog) GetEventLoggerNames() []string {
 	m.rwmutex.RLock()
@@ -460,3 +454,7 @@ func (m *MultiChannelledLog) ResetLevel() Level {
 func (m *MultiChannelledLog) GetName() string {
 	return m.name
 }
+
+func (e *Event) GetMsg() string {
+	return e.msg
+}