support more items inside #[defmt_test::tests]

[japaric]

currently, the #[tests] module can only contain function items that have one of these attributes: #[test] on #[init].
instead, we should support any kind of item that a normal module can contain: non-unit-test (e.g. helper) functions, structs, etc.

#[defmt_test::tests]
mod tests {
    #[test]
    fn f() {
        helper();
    }

    fn helper() {
        // ..
    }

    struct Helper;

    #[test]
    fn g() {
        Helper::constructor().method();
    }
}

implementation wise, items that are not unit tests or the #[init] function should not be touched by the proc-macro and emitted as they appear in the input