[bugfix][test] Fix functional test

While I'd like the tested interface more, it's not implemented
yet leading to a failing test

dune/tnnmg/functionals/test/functionaltest.hh

@@ -399,7 +399,8 @@ void testDirectionalRestrictionSubdifferential(const Functional& functional,
       for (auto v : testParameters)
       {
         // Test the subdifferential of the restriction
-        Solvers::Interval<double> subDifferential = restriction.subDifferential(v);
+        Solvers::Interval<double> subDifferential;
+        restriction.subDiff(v, subDifferential);
 
         // Step size. Best value: square root of the machine precision
         constexpr double eps = std::sqrt(std::numeric_limits<double>::epsilon());