Fix non-terminating regexp in roundtrip-test.js

* The same comment regexp that caused trouble in the WTS existed
  in normalizeNewlines function in mediawiki.Util.js for a while.
  Looks like it got exercised recently and sent roundtrip-test.js
  into a coma.

  Wikipedia:Articles for deletion/Wynnton is one of the many pages
  in the Wikipedia namespace that ran into trouble.

* The regexp in question is: "<!--(?:[^-]+|-(?!->))*-->"
  Looks like the "+" match inside combined with the "*" outside sent
  the regexp into an infinite loop. Converting it into the (a|b)*
  form where a and b match exactly one char fixes it.  This is the
  same fix in all the other instances. Just documenting here for
  posterity with a little more detail.

Change-Id: I30acde6cc4e882061137a87d0ea523ce76066ac5