How to handle an expected exception in base postgres

I want to be able to write a .sql script that will carry out an action which fails, and the script should only report failure if the action doesn't fail.

For example, given initial table:

create table tbl(x integer, y integer);

I might update this table in a migration with the following:

alter table tbl add constraint unique_tst unique (x, y); 

And want to write a test script for that migration, which will be similar to the following:

insert into tbl(x, y) values 
(1, 1),
(1, 1)
; 

This will fail - which is expected given the constraint - but I'm not sure how to handle that failure in postgres.

Something such as:

if does not fail: 
    insert into tbl(x, y) values 
    (1, 1),
    (1, 1)
    ; 
return:
    failure

But I have no idea if this exists.

Note - I cannot install any extensions for this.