testing-guides: add a command helping to identify the desired sink

Sometimes, "pactl list short sinks" doesn't give enough information
to determine which sink we want to use, then we have to use additional
information.

Signed-off-by: Dylan Aïssi <dylan.aissi@collabora.com>

content/qa/testing-guides.md

@@ -73,6 +73,11 @@ $ pactl list short sinks
 75	alsa_output.platform-sound.stereo-fallback	PipeWire	s16le 2ch 48000Hz	SUSPENDED
 ```
 
+If in doubt, the following command gives more information to identify the desired sink:
+```
+$ pactl list cards
+```
+
 After that you it is possible to set the default sink and port by:
 
 ```