Add an option for qmltestrunner to load translation files.

This is virtually a backport of
qtdeclarative/0339d34691ecf4d67d4ef41c028a687251c2d81a.

Done-with: Richard Braakman <richard.braakman@jollamobile.com>
Change-Id: I8d32c42536aafae1570b273655f200e037582948
Reviewed-by: Matthew Vogt <matthew.vogt@qinetic.com.au>
Reviewed-by: Robin Burchell <robin+qt@viroteck.net>