[tests] Add a new test for constexpr GeometryType

dune/geometry/test/CMakeLists.txt

@@ -23,3 +23,6 @@ dune_add_test(SOURCES test-nonetype.cc
 
 dune_add_test(SOURCES test-refinement.cc
               LINK_LIBRARIES dunegeometry)
+
+dune_add_test(SOURCES test-constexpr-geometrytype.cc
+              LINK_LIBRARIES dunegeometry)

dune/geometry/test/test-constexpr-geometrytype.cc

+#include <config.h>
+
+#include <iostream>
+
+#include <dune/geometry/type.hh>
+
+int main ( int argc, char **argv )
+{
+  constexpr auto gt_none_1 = Dune::GeometryType();
+  return not std::integral_constant<bool,gt_none_1.isNone()>{};
+}