Add nameFrom: heading tests once the AccName change is reviewed

[cookiecrook]

Write WPT label tests for ARIA/AccName "nameFrom: heading"

ARIA: w3c/aria#1860
AccName: w3c/accname#182

Circular process: I needed to file this WPT issue in order to merge w3c/aria#1860 but we can't write this test until the related AccName change w3c/accname#182 is reviewed or lands. Heh.

[cookiecrook]

ARIA Spec PR #1860 is approved. AccName Issue #182 is still outstanding.

Implementations mostly unblocked now, so we can proceed with this initial set of tests.

[cookiecrook]

AccName PR is now pending approved:

• Add nameFrom: heading to AccName text alt algorithm w3c/accname#229

[cookiecrook]

HTML-AAM change is coming too.

• Addition: Name from heading w3c/html-aam#529