Bug 398825 - Make the selectSubString method pref enabled and fix and re-enable the mochitests r=Ms2ger

--HG--
rename : content/svg/content/test/test_selectSubString.xhtml => content/svg/content/test/test_selectSubString2.xhtml
Bug 849239 - Remove leftover caught exception print()s from js tests, to avoid TBPL false positives; r=terrence
Back out 80034b418a48 (bug 844331, part 6) on suspicion of breaking Telemetry submission; r=vladan