[tests] use grid provider's refine method instead of manual

this solves problems in new dune-alugrid code, that
actually mandates prepost adapt calls

dune/gdt/test/operators/darcy.hh

@@ -54,8 +54,8 @@ struct DarcyOperatorTest : public ::testing::Test
   void produces_correct_results() const
   {
     GridProviderType grid_provider(0.0, 1.0, 4);
+    grid_provider.global_refine(1);
     auto& grid = grid_provider.grid();
-    grid.globalRefine(1);
 
     typedef Stuff::Functions::Expression<EntityType, DomainFieldType, dimDomain, RangeFieldType, 1> FunctionType;
     const FunctionType source("x", "x[0] * x[1]", 2, "source", {"x[1]", "x[0]"});

dune/gdt/test/prolongations/base.hh

@@ -30,7 +30,7 @@ struct ProlongationOperatorsBaseGridHolder
   ProlongationOperatorsBaseGridHolder()
     : grid_provider_(0.0, 1.0, 2u)
   {
-    grid_provider_.grid().globalRefine(1);
+    grid_provider_.global_refine(1);
   }
 
   GridProviderType grid_provider_;