MM-23261 plugin stderr debug logs (#14166)

* explicitly assert panic as error log

* Revert "[MM-18150] plugin panic trace should not be lost (#13559)"

This reverts commit 5d928b4f94, while leaving the unit tests intact
and now asserting debug logs instead.

* missing license header

testlib/assertions.go

@@ -0,0 +1,33 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package testlib
+
+import (
+	"bytes"
+	"encoding/json"
+	"io"
+	"testing"
+)
+
+// AssertLog asserts that a JSON-encoded buffer of logs contains one with the given level and message.
+func AssertLog(t *testing.T, logs *bytes.Buffer, 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 {
+			continue
+		}
+
+		if log.Level == level && log.Msg == message {
+			return
+		}
+	}
+
+	t.Fatalf("failed to find %s log message: %s", level, message)
+}