Bug 1935570 [wpt PR 49561] - Rename the tests in webrtc/legacy to have optional flags, a=testonly

Automatic update from web-platform-tests
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.)

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wpt-commits: 0e93f3294eddf7a67dfa49161a16ca82cb377108
wpt-pr: 49561

Author: gsnedders