glib-macros/async_test: Fixes unwrap result and move context

[BiagioFesta]

Tested code:

#[glib::async_test]
async fn my_test() {
    use std::time::Duration;

    glib::timeout_future(Duration::from_secs(1)).await;
}

Before this PR

fn my_test() {
    let main_ctx = glib::MainContext::new();
    main_ctx
        .with_thread_default(move || {
            main_ctx
                .block_on(async {
                    use std::time::Duration;
                    glib::timeout_future(Duration::from_secs(1)).await;
                })
        })
}

error[E0308]: mismatched types
  --> src/main.rs:12:1
   |
12 | #[glib::async_test]
   | ^^^^^^^^^^^^^^^^^^^ expected `()`, found `Result<(), BoolError>`
   |
   = note: expected unit type `()`
                   found enum `Result<(), BoolError>`

AND

error[E0505]: cannot move out of `main_ctx` because it is borrowed
  --> src/main.rs:23:30
   |
21 |     let main_ctx = glib::MainContext::new();
   |         -------- binding `main_ctx` declared here
22 |     main_ctx
   |     -------- borrow of `main_ctx` occurs here
23 |         .with_thread_default(move || {
   |          ------------------- ^^^^^^^ move out of `main_ctx` occurs here
   |          |
   |          borrow later used by call
24 |             main_ctx
   |             -------- move occurs due to use in closure
   |
help: consider cloning the value if the performance cost is acceptable
   |
22 |     main_ctx.clone()
   |             ++++++++

After this PR

fn my_test() {
    let main_ctx = glib::MainContext::new();
    main_ctx
        .with_thread_default(|| {
            main_ctx
                .block_on(async {
                    use std::time::Duration;
                    glib::timeout_future(Duration::from_secs(1)).await;
                })
        })
        .expect("cannot set thread default main context for test")
}