Rename the tests in webrtc/legacy to have optional flags

These tests are, if I'm not mistaken, entirely testing behaviour in
https://w3c.github.io/webrtc-pc/#legacy-interface-extensions, which
opens with:

> Supporting the methods in this section is optional. However, if
> these methods are supported it is mandatory to implement according
> to what is specified here.

Thus the behaviours tested within these tests are not mandatory tests,
and thus they must be marked as optional. (Arguably, they could be
made to be mandatory tests, if they simply allow for the overload not
being supported, but that's much harder to achieve.)

Author: gsnedders