Bug 1726416 - Update testcase for Arabic subtending marks (from bug 1599841)...

Bug 1726416 - Update testcase for Arabic subtending marks (from bug 1599841) now that harfbuzz handles the number-directionality issue internally. r=emilio

We can no longer use direction overrides to create the test vs reference renderings here,
because of how harfbuzz now handles the numeral directionality internally. So it's difficult
to come up with a testcase that verifies the exact rendering of the subtending mark + digit(s)
combinations.

What we can do, though, is verify that a single digit combines with the subtending mark
without affecting its overall width; and that a double-digit number *does* increase the
width. These two conditions are unlikely to both hold unless the rendering is behaving
correctly.

(FWIW, Chrome Canary passes these new test versions, as it also uses a recent harfbuzz;
Safari currently fails, as Core Text doesn't support the subtending marks in the same
way, if at all.)

Differential Revision: https://phabricator.services.mozilla.com/D124902