Display wxSize less confusingly if any tests fail

Using "x" as separator between the components created confusion with
hexadecimal notation, resulting in confusing messages like "0x17==0x0"
that seemed to describe comparison of 2 numbers and not 2 wxSize
objects.

Using "*" is also more consistent with wxRect output format.

tests/asserthelper.cpp

@@ -49,7 +49,7 @@ std::ostream& operator<<(std::ostream& os, const wxColour& c)
 
 std::ostream& operator<<(std::ostream& os, const wxSize& s)
 {
-    os << s.x << "x" << s.y;
+    os << s.x << "*" << s.y;
 
     return os;
 }