Turn block into a closure in test macro

[Aaron1011]

The doc_test! macro defines two local variables (entries and doc),
which are then accessed from the provided block.

However, this only compiled due to a bug in rustc: substituted
metavariables cannot refer to names defined within the macro body. For
example, the following ode does not compile:

macro_rules! foo {
    ($block:expr) => {
        let mut bar = false;
        $block
    }
}

fn main() {
    foo!({bar = true});
}

In this case, the doc_test! macro was incorrectly allowed to compile
due to the presence of the #[tokio::test] macro on the enclosing
function. When the underlying compiler bug is fixed in
rust-lang/rust#75800,
this macro will stop compiling.

I've adjusted the macro to take in a closure instead of a block. This
closure is invoked with the entries and doc variables, allowing it
to compile with both old and new compilers.

If you have any questions about this change, feel free to ask me.

[CLAassistant]

All committers have signed the CLA.

[bartlomieju]

@Aaron1011 nice catch and thanks for the patch!

CC @SyrupThinker PTAL

[SyrupThinker]

Looks good to me

[bartlomieju]

LGTM