fix singular matrix in tests/test_semismooth_newton.py
fix singular matrix in tests/test_semismooth_newton.py

vlukes/sfepy,sfepy/sfepy,RexFuzzle/sfepy,vlukes/sfepy,RexFuzzle/sfepy,rc/sfepy,BubuLK/sfepy,sfepy/sfepy,sfepy/sfepy,lokik/sfepy,vlukes/sfepy,RexFuzzle/sfepy,RexFuzzle/sfepy,lokik/sfepy,rc/sfepy,lokik/sfepy,BubuLK/sfepy,lokik/sfepy,rc/sfepy,BubuLK/sfepy