Test for `sizes` attribute support on `img`.

I like these easy ones.

Presumably this will be landing everywhere at the same time as `picture`, so support for `sizes`/`srcset` on `source` can likely be assumed based on the overarching `picture` test that landed yesterday ( abc570790ec92321b488991eb42363883471dbf1 ). I’ll be keeping an eye on this whether I like it or not, though, so I can revisit with a separate test on `picture > source` if `sizes` rolls out to `img` and `source` separately in any UA.