Merge pull request #9044 from julienw/859376-fix-panning-test a=tests

Bug 859376 - [homescreen] fix unreliable test about panning r=crdlc(cherry picked from commit 577789c2e22a239411fa156a160fbe5c9131801c)

Conflicts:
	apps/homescreen/test/unit/grid_test.js
(cherry picked from commit f2d67d4a3bbfa112c80d45f870a48a321f637ea3)

Fixed tests too.

Conflicts:
	apps/homescreen/test/unit/grid_test.js