Fix: Do not test with exact solution as initial iterate.

dune/solvers/test/gssteptest.cc

@@ -78,14 +78,14 @@ struct GSTestSuite {
     };
 
     auto test = [&](Step* step, std::string name) {
-      auto result = solve(step, p.u_ex, 1e-14, 2000);
+      auto result = solve(step, 0.0, 1e-14, 2000);
       passed &= analyse(result, name, 1e-8);
     };
 
     auto diffTest = [&](Step* step1, std::string name1, Step* step2,
                         std::string name2, double maxDiff = 0.0) {
-      auto result1 = solve(step1, p.u_ex, 0.0, 5);
-      auto result2 = solve(step2, p.u_ex, 0.0, 5);
+      auto result1 = solve(step1, 0.0, 0.0, 5);
+      auto result2 = solve(step2, 0.0, 0.0, 5);
 
       auto normDiff = p.energyNorm.diff(result1, result2);
       if (normDiff > maxDiff) {