tests: add check_output test helper

[bartlomieju]

Towards #803

[jedel1043]

Maybe it would be good to allow the execution of intermediate instructions between tests. That way we could cleanup some tests like:

let mut context = Context::new();
let init = r#"
    var iterator = [1, 2, 3].keys();
    var next = iterator.next();
"#;
forward(&mut context, init);
assert_eq!(forward(&mut context, "next.value"), "0");
assert_eq!(forward(&mut context, "next.done"), "false");
forward(&mut context, "next = iterator.next()");
assert_eq!(forward(&mut context, "next.value"), "1");
assert_eq!(forward(&mut context, "next.done"), "false");

I'd suggest creating a TestAction enum with the variants TestEq((&str, &str)) and Execute(&str). This would change the previous test to:

let init = r#"
    var iterator = [1, 2, 3].keys();
    var next = iterator.next();
"#;
check_output(
    &[
        TestAction::Execute(init),
        TestAction::TestEq(("next.value", "0")),
        TestAction::TestEq(("next.done", "false")),
        TestAction::Execute("next = iterator.next()"),
        TestAction::TestEq(("next.value", "1")),
        TestAction::TestEq(("next.done", "false")),
    ]
)

Notice how we don't need the maybe_init parameter, we just pass it as a statement to be executed.

[bartlomieju]

Sure, I can rework it to your suggestion

[bartlomieju]

@jedel1043 I applied your suggestion, please let me know if this is satisfactory. Once everyone's happy with this helper I can migrate more cases to use it (maybe across several PRs as there are ~1500 occurrences).

[HalidOdat]

This looks really good! We can merge this as is and refactor more in future PRs

[jedel1043]

Just a small nitpick about the test counter. Everything else is pretty good! We should be able to extend TestAction in case we find more common use cases.

[jedel1043]

Looks great to me!