Set the same dom-diff flag that WTS checks for!!

* After a 4-hour wild-goose chase confusing myself and looking at
  all the wrong places, I finally found that DOMDiff was setting
  'modified-children' flag and WTS (via DOMUtils) was looking for
  'children-changed' flag (see commit e20fec628).

* This fixes 12 more selser tests (and fixes the unrelated selser
  failures seen in https://gerrit.wikimedia.org/r/#/c/77245/)

Change-Id: I3b5794d03c76215bfac02ab708fd7d64bd87fd19