[MM-61460] Fix errcheck issues in server/channels/app/plugin_signature_test.go (#29348)

Co-authored-by: Ben Schumacher <ben.schumacher@mattermost.com>

server/.golangci.yml

@@ -107,7 +107,6 @@ issues:
         channels/app/platform/web_conn.go|\
         channels/app/platform/web_hub.go|\
         channels/app/platform/web_hub_test.go|\
-        channels/app/plugin_signature_test.go|\
         channels/app/plugin_test.go|\
         channels/app/post_helpers_test.go|\
         channels/app/post_test.go|\

server/channels/app/plugin_signature_test.go

@@ -42,7 +42,8 @@ func TestPluginPublicKeys(t *testing.T) {
 	fileReader, err := os.Open(filepath.Join(path, publicKeyFilename))
 	require.NoError(t, err)
 	defer fileReader.Close()
-	th.App.AddPublicKey(publicKeyFilename, fileReader)
+	appErr := th.App.AddPublicKey(publicKeyFilename, fileReader)
+	require.Nil(t, appErr)
 	file, appErr := th.App.GetPublicKey(publicKeyFilename)
 	require.Nil(t, appErr)
 	require.Equal(t, publicKey, file)