Testconfig: Fix QtWebEngine's Mac stage's PATH

QtWebEngine seems to require Python 2.7. This fixes the PATH so
that it find s the correct version of Python installation. The
default PATH seems to contain wrong version of Python.

Task-number: QTQAINFRA-740
Change-Id: I4997147099d80fb8000d7253376ae33d8d38ff69
Reviewed-by: Zeno Albisser <zeno.albisser@digia.com>