func checkLogForSeverity(s int, b []byte) error {
	expected := regexp.MustCompile(fmt.Sprintf(`^%s: [0-9]{4}/[0-9]{2}/[0-9]{2} [0-9]{2}:[0-9]{2}:[0-9]{2} %s\n$`, severityName[s], severityName[s]))
	if m := expected.Match(b); !m {
		return fmt.Errorf("got: %v, want string in format of: %v", string(b), severityName[s]+": 2016/10/05 17:09:26 "+severityName[s])
	}
	return nil
}
check if b is in the format of:
WARNING: 2017/04/07 14:55:42 WARNING
package grpclog
import (
	"bytes"
	"fmt"
	"regexp"
	"testing"
)
func TestLoggerV2Severity(t *testing.T) {
	buffers := []*bytes.Buffer{new(bytes.Buffer), new(bytes.Buffer), new(bytes.Buffer)}
	SetLoggerV2(NewLoggerV2(buffers[infoLog], buffers[warningLog], buffers[errorLog]))
	Info(severityName[infoLog])
	Warning(severityName[warningLog])
	Error(severityName[errorLog])
	for i := 0; i < fatalLog; i++ {
		buf := buffers[i]
		for j := i; j < fatalLog; j++ {
			b, err := buf.ReadBytes('\n')
			if err != nil {
				t.Fatal(err)
			}
			if err := checkLogForSeverity(j, b); err != nil {
				t.Fatal(err)
			}
		}
	}
}