Enforce the use of test.source position plug-in in auto tests

This avoids that platform specific plug-ins such as the geoclue plug-in
on Linux interfere with auto tests. The auto tests require the specific
test plug-in values to succeed.

Task-number: QTBUG-40702
Change-Id: I1c3ea9f6d23e1ad20815a86ded7815e498016af2
Reviewed-by: Aaron McCarthy <mccarthy.aaron@gmail.com>