[ACSL] fixes casts to unsigned short int (add test file)

tests/spec/cast_int.i

+unsigned short int toto;
+
+/*@ensures toto==(unsigned short int)param;*/
+int F(int param){
+return 0;
+}

tests/spec/oracle/cast_int.res.oracle

+[kernel] Parsing tests/spec/cast_int.i (no preprocessing)
+/* Generated by Frama-C */
+unsigned short toto;
+/*@ ensures toto ≡ (unsigned short)\old(param); */
+int F(int param)
+{
+  int __retres;
+  __retres = 0;
+  return __retres;
+}
+
+