Fix Merge label map strict test

The label map was overlapping, which causes a race condition when
converting to a regular image. This produced occasional test failures.

Change-Id: I62ccca178a92aae264480c6ad58a1fe33d9e1fb6
Merge topic 'SIMPLEITK-298_AddMergeLabelMapFilter' into next

858d7045 Fix Merge label map strict test