Always clean tested application environment and add option to preserve.

[scott-gibson-sociomantic]

Currently the environment logic is unclear exactly what is going on.

If the test suite returns a null env then the env of the test suite will be passed on to the spawned process.
Setting any env variable in the configureTestedApplication method will then cause the existing env to not be used. I recently discovered this when adding stomping prevention for d2 conversion tests and my path was now wiped, causing a test failure.

Behaviour should be updated to always clean the environment by default with an option to preserve.