Bug 1840845 enable setSinkId for dom/media/test/test_setSinkId_after_loop.html on beta r=padenot

This is already enabled by default on Nightly.
Apparently the pref needs to be set before the test Window is created.

Differential Revision: https://phabricator.services.mozilla.com/D182431

dom/media/test/mochitest.ini

@@ -26,6 +26,7 @@ subsuite = media
 skip-if = (os == "win" && processor == "aarch64") # aarch64 due to 1536604
 prefs =
   gfx.core-animation.low-power-telemetry-frames=60 # only for test_video_low_power_telemetry.html
+  media.setsinkid.enabled=true # for test_setSinkId_after_loop.html
 support-files =
   16bit_wave_extrametadata.wav
   16bit_wave_extrametadata.wav^headers^

dom/media/test/test_setSinkId_after_loop.html

@@ -34,7 +34,6 @@ add_task(async () => {
   });
 
   await SpecialPowers.pushPrefEnv({set: [
-    ["media.setsinkid.enabled", true],
     // skip gUM permission prompt
     ["media.navigator.permission.disabled", true],
     // enumerateDevices() without focus