[solverfactory] It unconditionally also tests unavailable solvers

I just wanted to deactivate SuiteSparse because there are problems on 32bit Debian. Problem is that UMFPack (and other things?) still gets tested and the tests fail:

 ============== UMFPack ============== 
terminate called after throwing an instance of 'Dune::InvalidStateException'
  what():  Dune::InvalidStateException [get:./dune/istl/solverfactory.hh:185]: Solver not found in the factory.
[arm-conova-02:2134729] *** Process received signal ***
[arm-conova-02:2134729] Signal: Aborted (6)
[arm-conova-02:2134729] Signal code:  (-6)
[arm-conova-02:2134729] *** End of error message ***

        Start 104: solverfactorytest_DuneFieldVectorDuneLoopSIMDdouble41
104/124 Test #104: solverfactorytest_DuneFieldVectorDuneLoopSIMDdouble41 .............Subprocess aborted***Exception:   0.58 sec