imac built-in mic test

It depends what you mean by testing. You can use the "Sound" part of the System Settings app, select "Input" and you can speak to see if Input level meter registers your voice (I don't have a Ventura system handy to verify this is still available). Or you can use Photobooth and record a video where you can speak if you actually want to verify the audio is correct.

Edit: The Apple Diagnostics may perform an audio test which will also verify the Mic by making sure the audio test played.

You can do a Zoom or Skype test call, too.