Move operator<<(std::ostream&, wxRect) overload to a header

This will allow reusing it in other tests too.

Also make the output slightly more readable by formatting the rectangle
as "{x,y w*h}" instead of "{x,y,w,h}".

No real changes.

tests/asserthelper.h

@@ -35,5 +35,6 @@ std::ostream& operator<<(std::ostream& os, const wxColour& c);
 std::ostream& operator<<(std::ostream& os, const wxSize& s);
 std::ostream& operator<<(std::ostream& os, const wxFont& f);
 std::ostream& operator<<(std::ostream& os, const wxPoint& p);
+std::ostream& operator<<(std::ostream& os, const wxRect& r);
 
 #endif