What is "FQC" test app in Nokia 6?
Just quit the app and it's ok.

If you change some settings (e.g in *#*#4636#*#*, setting priority to "Only GSM" or "WCDMA"), this will be set on your device. But if you only run it without changing the settings, quitting is enough.

To get your phone on "running test mode" you have to do it on "fbootloader" screen and set it to "factory mode", but restarting the device will get out of that.