COMP: Adjust error threshold for TestDensifyPolyData

Some platforms report image errors with small differences. Here we
increase the error threshold for TestDensifyPolyData. Also, the
CMakeLists.txt file is instrumented to allow easy addition of new
error thresholds.

Converted TestDensifyPolyData to use the new testing framework. This
permits error tolerances to be specified in the CMakeLists.txt file
rather than hard coded in the test.

Change-Id: I7759465def406ccd0f761e3e478dd754806e084e
Merge topic 'TestDensifyPolyData_tolerance' into master

9abd425f COMP: Adjust error threshold for TestDensifyPolyData