Fix mis-implemented test

[masak]

Just noticed that this test doesn't test the right thing.

{
    my $program = q:to/./;
        func infix:<n>(left, right) {
            return 20;
        }
        say(4 * 5);
        .

    outputs $program, "20\n", "using an operator after defining it works";
}

It's not using the defined n operator, it's using the built-in *, which is quite uninteresting.