Fix hygiene of test! macro

[Aaron1011]

When writing a `macro_rules! macro, any variables declared within the
right-hand side of a macro will inacessible to code expanded from a
metavariable. For example, the following code produces an error:

macro_rules! use_it {
    ($code:block) => {
        let a = "25";
        $code
    }
}

fn main() {
    use_it!({
        let b = a;
    });
}

Since 'let b = a' cannot access the 'a' variable defined within 'use_it'.

Due to a bug in the Rust compiler, this type of code was sometimes
when a proc-macro expansion occured near a macro_rules! expansion.

In this crate, the test! macro defines a local variable it, which is
then accessed from the $steps:block metavaraible.

This code will stop compiling in a future version of Rust (specifically,
once PR rust-lang/rust#77153 is merged).

I've adjusted the test! macro to take in a $it:ident metavariable,
which is used as the name of the local variable. To reduce churn, I've
made this optional - only one use of the test! macro needed to be
updated.