feat(JitsiConference) Allow adding multiple tracks to the conference.

This should be supported by the backend. This limitation was added here only because of track synchronization issues between JM and LJM. The support is added behind testing.allowMultipleTracks config.js setting
Fixes https://github.com/jitsi/lib-jitsi-meet/issues/2205

JitsiConference.js

 
         // Currently, only adding multiple video streams of different video types is supported.
         // TODO - remove this limitation once issues with jitsi-meet trying to add multiple camera streams is fixed.
-        if (mediaType === MediaType.VIDEO
-            && !localTracks.find(t => t.getVideoType() === track.getVideoType())) {
+        if (this.options.config.testing?.allowMultipleTracks
+            || (mediaType === MediaType.VIDEO && !localTracks.find(t => t.getVideoType() === track.getVideoType()))) {
             const sourceName = getSourceNameForJitsiTrack(
                 this.myUserId(),
                 mediaType,
             return Promise.all(addTrackPromises)
                 .then(() => {
                     this._setupNewTrack(track);
-                    this._sendBridgeVideoTypeMessage(track);
+                    mediaType === MediaType.VIDEO && this._sendBridgeVideoTypeMessage(track);
                     this._updateRoomPresence(this.getActiveMediaSession());
 
                     if (this.isMutedByFocus || this.isVideoMutedByFocus) {