improve management of test directories

I would move this up to a new static field next to `TEST_IN_SAME_PROCESS` (and actually declared in the same manner since it's also a `boolean` setting). However, since `Boolean.parseBoolean(null)` and `Boolean.parseBoolean("")` both return `false`, I would invert the semantics to `PRESERVE_TEMPORARY_OUTPUT_DIRECTORY` (instead of `ERASE_TEMPORARY_OUTPUT_DIRECTORY`) so users that don't define this property still get the current behavior.