Fix bugs in BrittleFractureLocalSolver

Sander, Oliver requested to merge fix-brittlefracture-localsolver into master

... and tighten the unit test. Out of lazyness, the old test used the zero function for the quadratic part of the functional, which promptly hid a few bugs. Adding a non-trivial quadratic part to the test makes it much more helpful.

Edited by Sander, Oliver

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