Test all backends in gitlabci

Closes #16 (closed)

test/DiscreteFunctionTest.cpp

@@ -300,6 +300,7 @@ void test2(Grid& grid, BasisFactory&& basis)
 template <class GridView>
 void test3(GridView const& gridView)
 {
+#if AMDIS_BACKEND == AMDIS_BACKEND_ISTL
   using namespace Dune::Functions::BasisFactory;
   auto blockedBasis = makeBasis(gridView, power<2>(lagrange<2>()));
 
@@ -316,6 +317,7 @@ void test3(GridView const& gridView)
 
   ISTLBlockVector<double> vec4(flatBasis);
   DiscreteFunction u4{vec4, flatBasis};
+#endif
 }