Update TestBench dependency to 3.1.2

TestBench 3.1.2 is deployed to the testing grid, but the vaadin
project still depended on 3.1.1 until this fix.

Change-Id: Idecf0c772bd59d144a2caf3454d7749e352fef8a
Merge commit '691580e36b1b5a04e1f18004783b28ca9892fa31' into master

691580e Update TestBench dependency to 3.1.2

Conflicts:
	uitest/ivy.xml

Change-Id: Ic2a767595791f0730a209d833f6b9d0fcaaa4efe