Fix building GUI tests with MSVC

This was broken by da48b9e45d (adding an iOS build to travis (#1847),
2020-05-08), as we can't use preprocessor directives inside macro
arguments with MSVC (at least with its traditional preprocessor).

Use a helper macro to make this work again.

tests/controls/dialogtest.cpp

@@ -59,13 +59,18 @@ void ModalDialogsTestCase::MessageDialog()
 {
     int rc;
 
+#if wxUSE_FILEDLG
+    #define FILE_DIALOG_TEST ,\
+        wxExpectModal<wxFileDialog>(wxGetCwd() + "/test.txt").Optional()
+#else
+    #define FILE_DIALOG_TEST
+#endif
+
     wxTEST_DIALOG
     (
         rc = wxMessageBox("Should I fail?", "Question", wxYES|wxNO),
         wxExpectModal<wxMessageDialog>(wxNO)
-#if wxUSE_FILEDLG
-        ,wxExpectModal<wxFileDialog>(wxGetCwd() + "/test.txt").Optional()
-#endif
+        FILE_DIALOG_TEST
     );
 
     CPPUNIT_ASSERT_EQUAL(wxNO, rc);