Consider adding 'tests' columns to tables

[scottaohara]

During the ARIA editor's meeting call we discussed potentially adding a column to each element/attribute table for tests.

Writing this issue to serve both as a reminder and sounding board for what this could actually look like, since each feature could have anywhere from one to several tests that could be linked.

If it was a one to one pairing, each HTML element has one test page that contains all tests relevant for that element, then it's rather straight forward in presentation. But this then this means current tests that contain many elements would need to be broken apart.

if current tests that contain multiple elements are kept, then it means that there would need to be multiple test links for each HTML feature. which is probably fine.

[jnurthen]

for a respec feature to expose this see https://github.com/w3c/respec/issues/4633