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

app/plugin_test.go

@@ -13,7 +13,6 @@ import (
 	"net/http/httptest"
 	"os"
 	"path/filepath"
-	"strings"
 	"testing"
 	"time"
 
@@ -21,8 +20,10 @@ import (
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 
+	"github.com/mattermost/mattermost-server/v5/mlog"
 	"github.com/mattermost/mattermost-server/v5/model"
 	"github.com/mattermost/mattermost-server/v5/plugin"
+	"github.com/mattermost/mattermost-server/v5/testlib"
 	"github.com/mattermost/mattermost-server/v5/utils"
 	"github.com/mattermost/mattermost-server/v5/utils/fileutils"
 )
@@ -665,8 +666,7 @@ func TestPluginPanicLogs(t *testing.T) {
 		_, err := th.App.CreatePost(post, th.BasicChannel, false)
 		assert.Nil(t, err)
 
-		logs := th.LogBuffer.String()
-		assert.True(t, strings.Contains(logs, "some text from panic"))
+		testlib.AssertLog(t, th.LogBuffer, mlog.LevelDebug, "panic: some text from panic")
 	})
 }