First chrome wpt test added

We've landed our first chrome wpt test which is for ConstantSourceNode,
because it's relatively self-contained.

You can see the results at
https://wpt.fyi/webaudio/chrome/the-constantsourcenode-interface

As discussed briefly on the teleconf, we do want to move these to the right
place eventually, but to do that we want to coordinate with all the browser
vendors first.

Thus, we'd like the vendors to take a look at the tests and see what you
think and whether we should move forward with these.  In particular, Chrome
has a wrapper around the wpt test methods to make it easier to write and
more informative when the tests fail.

The relevant change is
https://github.com/w3c/web-platform-tests/commit/fabf91dc28ca3e2ae83099b6b169fc16ce657b06