Remove last v8 dependency in the testlib

Change-Id: I68fa531a3db0becf0a449265562e8d20b9fa967f
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>
Allow construction of array with a predefined length

The execution engine was missing this method for a long
time. This should simplify some code and allow more optimised
paths in other places.

Change-Id: Iaf576beb3bc006cdcfd65b518a2121a13629f462
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>