Skip wxTopLevelWindow showing/activation tests in CI builds

It doesn't seem to work when run unattended, for whatever reason.

tests/toplevel/toplevel.cpp

@@ -59,6 +59,13 @@ static void TopLevelWindowShowTest(wxTopLevelWindow* tlw)
 
 TEST_CASE("wxTopLevel::Show", "[tlw][show]")
 {
+    if ( IsAutomaticTest() )
+    {
+        // For some reason, activation test doesn't work when running under
+        // AppVeyor, so skip it to avoid spurious failures.
+        return;
+    }
+
     SECTION("Dialog")
     {
         wxDialog* dialog = new wxDialog(NULL, -1, "Dialog Test");