Make name-from-heading test file non-tentative once the ARIA PR is merged

[cookiecrook]

Make "name from heading" test file non-tentative once the ARIA PR is merged.

• Add namefrom: heading. w3c/aria#2650

Current test results here:
https://wpt.fyi/results/accname/name/comp_name_from_heading.tentative.html?label=experimental&label=master&aligned