Cleanup overlay ART configuration and add MT tests (ATLASSIM-3947)

This MR cleans-up overlay ART configuration in master:

• The package has been moved to Simulation/Tests/OverlayTests in the effort to move overlay away from Event base folder.
• Added full chain test (disabled for now).
• Added MT tests: sequential, 1 thread and 4 threads.

@tcuhadar, is it fine if the package is renamed? Also if I remove the art-include will the test be disabled?

FYI @abasalae, @jchapman, @tkharlam