STYLE: Fix style in miscellaneous FEM tests

[jhlegarreta]

• STYLE: Remove commented statements with hard-coded filenames in FEM
• STYLE: Start reader/writer FEM module local variables with lowercase

PR Checklist

• No API changes were made (or the changes have been approved)
• No major design changes were made (or the changes have been approved)

[jhlegarreta]

A few comments:

• The lines removed in e455f53 for itkFEMLandmarkLoadImplementationTest and itkFEMSolverTest3D had slightly different names when compared to the CMakeLists.txt test command, as the removed lines had filenames that replicated the test name (only for the output filename for the latter).
• This module is turned off by default, and despite issue itkFEMElement2DC0LinearQuadrilateralStressTestFEMObjectReader has hard-coded I/O filenames #4417, this module will (could) succeed in testing.