[Test] Add check to shrink_to_time method in grid function container

dune/dorie/test/test-grid-function-container.cc

@@ -143,15 +143,41 @@ bool test_container(GV& gv)
     failed |= true;
   } catch (...) {}
 
-  // Try again another valid time
+  // try again another valid time
   gf_container.setTime(1.0);
   gf_container.evaluate(entity,x,y);
   if (not Dune::FloatCmp::eq(y[0],1.0))
     failed |= true;
 
+  // shrink to some arbitrary time
+  gf_container.setTime(1.5);
+  gf_container.shrink_to_time();
+  gf_container.evaluate(entity,x,y);
+  failed |= gf_container.size()==2;
+  try {
+    gf_container.setTime(0.5);
+    gf_container.setTime(1.5);
+    failed |= true;
+  } catch (...) {}
+
+  // shrink to some exact contained time
+  gf_container.setTime(1.);
+  gf_container.shrink_to_time();
+  gf_container.evaluate(entity,x,y);
+  failed |= gf_container.size()==1;
+  try {
+    gf_container.setTime(0.5);
+    gf_container.setTime(1.5);
+    failed |= true;
+  } catch (...) {}
+
+  // pop until container is empty
   while (not gf_container.empty())
     gf_container.pop();
 
+  // check that container is void
+  failed |= gf_container.size() == 0;
+
   // evaluate an empty container should rise an error 
   try {
     gf_container.evaluate(entity,x,y);