make the test working dir configurable

The test working directory can be set via the environment variable
QBS_TEST_WORK_ROOT. On systems with path length restrictions
(Windows, what else...) the tests might fail if you run them in a
directory that's too deep.

Change-Id: Ia7af8fc2c7a13171551a240d9d58e8815ecc662c
Reviewed-by: Christian Stenger <christian.stenger@theqtcompany.com>
Reviewed-by: Christian Kandeler <christian.kandeler@theqtcompany.com>