Can assertion/1 be used with constraints?

Here is a simple valid constraint for a diagonal line

line(X,Y) :-
    X #= Y.

The constraint works as expected.

:- use_module(library(clpfd)).

?- line(X,Y).
X = '$VAR'('Y'),
'$VAR'('Y')in inf..sup.

?- findall((X,Y),(between(-5,5,X),line(X,Y)),Line).
Line = [(-5, -5),  (-4, -4),  (-3, -3),  (-2, -2),  (-1, -1),  (0, 0),  (1, 1),  (2, 2),  (3, 3),  (4, 4),  (5, 5)].

Now in writing test cases I find it easier to let the system output the correct answer and then I copy it into the test case, e.g.

:- begin_tests(simple_constraints).

test(1) :-
    X = 5,
    assertion( X == x ).

:- end_tests(simple_constraints).

When the test is run it outputs a result, which I then verify is the correct result before coping back into the test. In this case x = 5

ERROR: c:/xyz.pl:40:
        test 4: assertion failed
        Assertion: 5==x
:- begin_tests(simple_constraints).

test(1) :-
    X = 5,
    assertion( X == 5 ).

:- end_tests(simple_constraints).

and re-run the test.

?- make.
% c:/xyz compiled 0.00 sec, -2 clauses
% PL-Unit: simple_constraints ... done
% All 3 tests passed
true.

Doing the same for the constraint does not work.

:- begin_tests(simple_constraints).

test(2) :-
    line(X,Y),
    assertion( X == x ),
    assertion( Y == y ).

:- end_tests(simple_constraints).

Running of the test

?- make.
% c:/xyz compiled 0.00 sec, 2 clauses
% PL-Unit: simple_constraints ..
ERROR: c:/xyz.pl:35:
        test 3: assertion failed
        Assertion: _10244==x
ERROR: c:/xyz.pl:35:
        test 3: assertion failed
        Assertion: _10244==y
A. done
% 2 assertions failed
% 1 test failed
% 3 tests passed
true.

Instead of

_10244==x

I was hoping for something like

'$VAR'('Y') == x