include xdp test to testsuite

Author: jalvesz

test/string/test_string_to_number.fypp

@@ -17,6 +17,9 @@ contains
             new_unittest("to_dp", test_to_dp) &
 #:if WITH_QP
             , new_unittest("to_qp", test_to_qp) &
+#:endif
+#:if WITH_XDP
+            , new_unittest("to_xdp", test_to_xdp) &
 #:endif
             ]
     end subroutine collect_string_to_number