wpt test for more simulcast research

My starting point.

by jib1

HTML

<script src="https://jan-ivar.github.io/dummy/testharness.js"></script>
<script src="https://rawgit.com/web-platform-tests/wpt/master/webrtc/third_party/sdp/sdp.js"></script>
<script src="https://rawgit.com/web-platform-tests/wpt/master/webrtc/simulcast/simulcast.js"></script>

JavaScript

const offer2 = `v=0
o=- 3840232462471583827 2 IN IP4 127.0.0.1
s=-
t=0 0
a=group:BUNDLE 0
a=msid-semantic: WMS
m=video 9 UDP/TLS/RTP/SAVPF 96
c=IN IP4 0.0.0.0
a=rtcp:9 IN IP4 0.0.0.0
a=ice-ufrag:Li6+
a=ice-pwd:3C05CTZBRQVmGCAq7hVasHlT
a=ice-options:trickle
a=fingerprint:sha-256 5B:D3:8E:66:0E:7D:D3:F3:8E:E6:80:28:19:FC:55:AD:58:5D:B9:3D:A8:DE:45:4A:E7:87:02:F8:3C:0B:3B:B3
a=setup:actpass
a=mid:0
a=extmap:4 urn:ietf:params:rtp-hdrext:sdes:rtp-stream-id
a=recvonly
a=rtcp-mux
a=rtpmap:96 VP8/90000
a=rtcp-fb:96 goog-remb
a=rtcp-fb:96 transport-cc
a=rtcp-fb:96 ccm fir
a=rid:foo recv
a=rid:bar recv
a=rid:baz recv
a=simulcast:recv foo;bar
`;

const offer3 = offer2.replace('a=simulcast:recv foo;bar',
                               'a=simulcast:recv foo;bar;baz');
const offer1 = offer2.replace('a=simulcast:recv foo;bar',
                               'a=simulcast:recv foo');
const offer0 = offer2.replace('a=simulcast:recv foo;bar',
                               'a=noop');

promise_test(async t => {
  const pc = new RTCPeerConnection();
  t.add_cleanup(() => pc.close());
  const [track, stream] = await getTrackFromUserMedia('video');
  t.add_cleanup(() => track.stop());
  await pc.setRemoteDescription({type: 'offer', sdp: offer3});
  pc.addTrack(track, stream);
  await pc.setRemoteDescription({type: 'rollback'});
  assert_equals(pc.getTransceivers().length, 1, 'Expected exactly one transceiver');
  assert_equals(pc.getTransceivers()[0].sender.getParameters().encodings.length, 3, 'encodings survive');
//  console.log(JSON.stringify(pc.getTransceivers()[0].sender.getParameters()));
}, 'transceiver saved from rollback of sRD(simulcastOffer) by addTrack is simulcast');


promise_test(async t => {
  const pc = new RTCPeerConnection();
  t.add_cleanup(() => pc.close());
  const [track, stream] = await getTrackFromUserMedia('video');
  t.add_cleanup(() => track.stop());
  await pc.setRemoteDescription({type: 'offer', sdp: offer3});
  pc.addTrack(track, stream);
  await...