Check that the grid cell has the expected value in the test

In addition to checking that we get the expected events, also verify
that editing the cell actually worked.

tests/controls/gridtest.cpp

@@ -203,6 +203,8 @@ void GridTestCase::CellEdit()
     }
 #endif // __WXGTK__
 
+    CHECK(m_grid->GetCellValue(1, 1) == "abab");
+
     CPPUNIT_ASSERT_EQUAL(1, created.GetCount());
     CPPUNIT_ASSERT_EQUAL(1, changing.GetCount());
     CPPUNIT_ASSERT_EQUAL(1, changed.GetCount());