[a11y] Move disputed self/cross-referential aria-labelledby tests to tentative file

[nmlapre]

Closes https://github.com/web-platform-tests/interop-accessibility/issues/123

There is a pending spec discussion surrounding these tests - we're not sure if they're valid as written, according to the current spec. This commit moves them to a new tentative file pending resolution of that discussion.

If we'd rather fix these tests a different way, I'm happy to change this PR. I'm suggesting moving them to a tentative file because I think that the core of what they are intended to test is in dispute.

cc @dandclark @jcsteh @zcorpan

[dandclark]

I don't have reviewer permissions in this repo, but LGTM. Thanks!