Handle invalid wxColour better in unit test assertions

Show invalid colour as "invalid" instead of "" and don't trigger a wx
assertion when comparing with it.

tests/asserthelper.cpp

@@ -13,7 +13,7 @@
 
 std::ostream& operator<<(std::ostream& os, const wxColour& c)
 {
-    os << c.GetAsString(wxC2S_HTML_SYNTAX);
+    os << (c.IsOk() ? c.GetAsString(wxC2S_HTML_SYNTAX) : "invalid");
 
     return os;
 }

tests/graphics/colour.cpp

@@ -34,6 +34,9 @@ public:
 
     bool match(const wxColour& c) const override
     {
+        if ( !c.IsOk() )
+            return false;
+
         return c.Red() == m_red && c.Green() == m_green && c.Blue() == m_blue;
     }