Ensures phpunit run from the makefile respects MW_INSTALL_PATH

This makes it possible to rely on the makefile for running phpunit tests
in MobileFrontend on setups where MobileFrontend doesn't actually live
in a standard location.

Change-Id: Ie28dc4119b6e6d0a8f7e266db66529b17af55538
Merge "Ensures phpunit run from the makefile respects MW_INSTALL_PATH"