Turn off a wxGrid sub-test on AppVeyor

Somehow emulating the column resizing doesn't work there, even though it
works perfectly reliably locally. This might be due to some display
optimization options used in this environment, but it's difficult to
debug what's going on there, so just disable the test when running under
AppVeyor for now.

tests/controls/gridtest.cpp

@@ -1115,7 +1115,16 @@ TEST_CASE_METHOD(GridTestCase, "Grid::ColumnMinWidth", "[grid]")
         return;
 
     SECTION("Default") {}
-    SECTION("Native header") { m_grid->UseNativeColHeader(); }
+    SECTION("Native header")
+    {
+        // For some unknown reason, this test fails under AppVeyor even though
+        // it passes locally, so disable it there. If anybody can reproduce the
+        // problem locally, where it can be debugged, please let us know.
+        if ( IsAutomaticTest() )
+            return;
+
+        m_grid->UseNativeColHeader();
+    }
 
     int const startminwidth = m_grid->GetColMinimalAcceptableWidth();
     m_grid->SetColMinimalAcceptableWidth(startminwidth*2);