docs (QA): Add Sound page tests (#2099)

https://github.com/pop-os/cosmic-settings/issues/2097 really should have
been caught by QA. It appears only the standard regression testing
checklist was performed for
https://github.com/pop-os/cosmic-settings/pull/2063, even though that PR
only modified the Sound page, and we had nothing in the standard
checklist for the Sound page.

To rectify that, this adds a few basic smoke tests for the Sound page in
the standard checklist, to hopefully prevent this from happening again.
(I know we've had this particular problem pop up at least once before,
too, so both sliders working independently is a good thing to have in
the checklist.)

---

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TESTING.md

@@ -48,7 +48,10 @@ This document provides a regression testing checklist for COSMIC Settings and th
 
 ### Sound
 
-TBD after devices/profiles are fixed.
+- [ ] Output & Input volume sliders work independently
+- [ ] Output balance slider works & doesn't get reset by volume slider
+- [ ] Amplification toggles work as expected
+- [ ] Device profiles can be changed & reflect as expected on the main Sound page
 
 ### Power & Battery