Refactor integration tests by reusing godot_test macro

[Bromeon]

Reduces boilerplate of the form

fn test_with_certain_name() -> bool {
    println!(" -- test_with_certain_name");

    let ok = std::panic::catch_unwind(|| {
        // actual test code here
    })
    .is_ok();

    if !ok {
        godot_error!("   !! Test test_with_certain_name failed");
    }

    ok
}

to

godot_itest! { test_with_certain_name {
    // actual test code here
}}

which reduces repetition and focuses on important things. This is done by slightly adjusting the existing godot_test macro to godot_itest, which is needed so it can also be used in the tests crate.

This change leads to a net removal of 300 lines of code.

---

I was also considering a proc-macro attribute

#[godot_test]
fn test_with_certain_name() {
    // actual test code here
}

which might be a tiny bit nicer syntax-wise, and even started implementing it. But I think the declarative macro does the job, is simpler and likely faster to compile (as we don't need syn to tokenize the entire code).

---

Tests of this form now all have the #[must_use] attribute, yielding a warning if a boolean test result is ignored.

[Bromeon]

bors r+

[bors]

Build succeeded:

• full-ci

[Bromeon]

Clippy nightly failed due to a regression in Clippy: rust-lang/rust-clippy#8867