Compare minima for the sample function

src/test-gradient-method.cc

@@ -60,7 +60,13 @@ void testSampleFunction() {
   error -= returned;
   assert(error.two_norm() < 1e-10); // FIXME: 1e-10 sounds reasonable. Is it?
 
-  functionTester(J, start, 6);
+  double const ret1 = functionTester(J, start, 6);
+
+  start[0] = 279;
+  start[1] = -96;
+
+  double const ret2 = functionTester(J, start, 10);
+  assert(std::abs(ret1 - ret2) < 1e-5);
 }
 
 void testSampleFunctionNonsmooth() {