[CI] Enable audio tests in Firefox (#23701)

Following on from #23665, a dummy audio device is now enabled for
Firefox in the CI.

Multiple attempts were tried (adding Alsa's `snd-dummy` and
`snd-aloop`), eventually getting PulseAudio to work.

The output from the Firefox runs was manually verified that the tests
had run (if they hadn't, due to work being run in the AudioWorklet
callback, they'd have timed out and failed).

Author: cwoffenden

.circleci/config.yml

@@ -353,15 +353,26 @@ commands:
           command: |
             wget -O ~/ff.tar.bz2 "https://download.mozilla.org/?product=firefox-nightly-latest-ssl&os=linux64&lang=en-US"
             tar -C ~ -xf ~/ff.tar.bz2
+      - run:
+          name: Add audio dependencies
+          command: |
+            # This should add and start PulseAudio's dummy mixer. It will warn
+            # that "This program is not intended to be run as root" but it can
+            # be ignored.
+            apt-get update -y
+            apt-get install -q -y pulseaudio
+            pulseaudio --start
       - run:
           name: configure firefox
           command: |
+            # Note: the autoplay pref allows playback without user interaction
             mkdir ~/tmp-firefox-profile/
             cat > ~/tmp-firefox-profile/user.js \<<EOF
             user_pref("gfx.offscreencanvas.enabled", true);
             user_pref("javascript.options.shared_memory", true);
             user_pref("javascript.options.wasm_memory64", true);
             user_pref("dom.postMessage.sharedArrayBuffer.bypassCOOP_COEP.insecure.enabled", true);
+            user_pref("media.autoplay.default", 0);
             EOF
       - run:
           name: run tests (<< parameters.title >>)
@@ -371,8 +382,6 @@ commands:
             #       support in headless mode) resolves
             EMTEST_LACKS_GRAPHICS_HARDWARE: "1"
             EMTEST_LACKS_WEBGPU: "1"
-            # TODO: replicate with FF the same as CHROME_FLAGS_AUDIO
-            EMTEST_LACKS_SOUND_HARDWARE: "1"
             # OffscreenCanvas support is not yet done in Firefox.
             EMTEST_LACKS_OFFSCREEN_CANVAS: "1"
             EMTEST_DETECT_TEMPFILE_LEAKS: "0"