Add nameFrom: heading tests once the AccName change is reviewed

Write WPT label tests for ARIA/AccName "nameFrom: heading" 

ARIA: https://github.com/w3c/aria/pull/1860
AccName: https://github.com/w3c/accname/issues/182

Circular process: I needed to file this WPT issue in order to merge https://github.com/w3c/aria/pull/1860 but we can't write this test until the related AccName change https://github.com/w3c/accname/issues/182 is reviewed or lands. Heh.