Switch to BoxMarker in point_caching_adaptive_refinement test.

Also displacing the Dirac points off the element corners slightly, to
make this example more robust in parallel.

Refs #4442.
Merge pull request #4452 from jwpeterson/dirac_adapt

Switch to BoxMarker in point_caching_adaptive_refinement test.
Merge commit '526a78964e508c7395615c207b52597b33935a26'