Put quotes around strings in Catch::StringMaker<wxString>

This makes the output in case of test failure more readable, especially
if one of the strings is empty.

include/wx/catch_cppunit.h

@@ -89,14 +89,16 @@ namespace Catch
         static std::string convert(const wxString& wxs)
         {
             std::string s;
-            s.reserve(wxs.length());
+            s.reserve(wxs.length() + 2);
+            s += '"';
             for ( auto c : wxs )
             {
                 if ( c >= 128 || !iswprint(c) )
                     s += wxString::Format(wxASCII_STR("\\u%04X"), c).ToAscii();
                 else
                     s += c;
             }
+            s += '"';
 
             return s;
         }