[Testing] Modularize functions in ForceFieldTestCreation and add test of buildStiffnessMatrix

[alxbilger]

• Refactor ForceFieldTestCreation: modularize functions for clarity
• Enhance ForceFieldTestCreation: add checkBuildStiffnessMatrix and refactor stiffness matrix checks

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[alxbilger]

I mark it as breaking as the change on the call of getPotentialEnergy requires this method to be exposed in all tested classes.