Refactor MockPhysicsManager

- Rename Testing::PhysicsManager to Testing::MockPhysicsManager
- Add doxygen about class's purpose
Fem3DPerformanceTest, remove mention of truth cube
Fem3DPerformanceTest refactor, use parameterized testing

- Use parameterized testing for integration scheme, and include all
integration schemes.
- Use parameterized testing for number of cube nodes, and test over
a variety of nodes
- Add xml output properties for the integration scheme and number of
nodes.