MM-61524: Fix flaky test RedisPubSub (#29311)

- We refactor some of the testlib assertion code
and add a new function to just return true or false.

https://mattermost.atlassian.net/browse/MM-61524
```release-note
NONE
```

server/channels/testlib/assertions.go

@@ -7,33 +7,33 @@ import (
 	"encoding/json"
 	"io"
 	"testing"
+
+	"github.com/stretchr/testify/assert"
 )
 
 // AssertLog asserts that a JSON-encoded buffer of logs contains one with the given level and message.
 func AssertLog(t *testing.T, logs io.Reader, level, message string) {
-	dec := json.NewDecoder(logs)
-	for {
-		var log struct {
-			Level string
-			Msg   string
-		}
-		if err := dec.Decode(&log); err == io.EOF {
-			break
-		} else if err != nil {
-			t.Logf("Error decoding log entry: %s", err)
-			continue
-		}
-
-		if log.Level == level && log.Msg == message {
-			return
-		}
+	t.Helper()
+	if !hasMsg(t, logs, level, message) {
+		assert.Failf(t, "failed to find %s log message: %s", level, message)
 	}
-
-	t.Fatalf("failed to find %s log message: %s", level, message)
 }
 
 // AssertNoLog asserts that a JSON-encoded buffer of logs does not contains one with the given level and message.
 func AssertNoLog(t *testing.T, logs io.Reader, level, message string) {
+	t.Helper()
+	if hasMsg(t, logs, level, message) {
+		assert.Failf(t, "found %s log message: %s", level, message)
+	}
+}
+
+// CheckLog checks whether a JSON-encoded buffer of logs contain the given
+// message at the level or not.
+func CheckLog(t *testing.T, logs io.Reader, level, message string) bool {
+	return hasMsg(t, logs, level, message)
+}
+
+func hasMsg(t *testing.T, logs io.Reader, level, message string) bool {
 	dec := json.NewDecoder(logs)
 	for {
 		var log struct {
@@ -48,8 +48,8 @@ func AssertNoLog(t *testing.T, logs io.Reader, level, message string) {
 		}
 
 		if log.Level == level && log.Msg == message {
-			t.Fatalf("found %s log message: %s", level, message)
-			return
+			return true
 		}
 	}
+	return false
 }