atlas_add_test: Add PRIVATE_WORKING_DIRECTORY option

Add option to run the test in a private working directory. This avoids users having to write this code manually and also standardizes the location of these private test directories. Private working directories are always deleted before running the test to avoid left-overs during incremental or local builds.

Some examples where this will be useful:

• https://gitlab.cern.ch/atlas/athena/-/blob/master/Trigger/TriggerCommon/TriggerMenuMT/CMakeLists.txt
• https://gitlab.cern.ch/atlas/athena/-/blob/master/Control/AthenaMonitoringKernel/CMakeLists.txt

TODO:

• Update https://twiki.cern.ch/twiki/bin/viewauth/AtlasComputing/SoftwareDevelopmentWorkBookCMakeInAtlas