Speaker Test popover: no button for mono

I have a Steelseries Arctis 7 headset, it features both stereo (main) and mono (for voice chat) outputs with a volume balance dial on headset.
It works in mono out of the box (it needs additional PulseAudio profiles for stereo and mono: [1](https://www.reddit.com/r/steelseries/comments/6bah8g/arctis_7_linux_support/), [2](https://baldpenguin.blogspot.com/2017/08/steeleries-arctis-7-headset-in-linux.html)). It would be nice if it worked right out of the box, but it's probably PulseAudio problem.

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There is a tiny UI bug: it's impossible to test mono output in Speaker Test popover.

#### Stereo:

#### Mono:

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