fix: make assertion assert something (#6768)
### Description
`assert_matches!` is a very explicit macro that expands
`assert_matches!(x, y)` to `match x { y => (), ref other => panic!("got
{other:?"})`. The `y` branch will be taken every time in this example
since`y` refers to a named variable and *not* a bound `y` that might
exist. See [matching named
variables](https://doc.rust-lang.org/book/ch18-03-pattern-syntax.html#matching-named-variables)
from the Rust book.
We had `_expected` instead of `expected` because clippy would warn us
that the variable was never used and it was right! The the `_expected`
in the `assert_matches!` was referring to a different variable than the
`_expected` defined above it.
This PR changes the `assert_matches` to use an variant so the assertion
is checking what we want.
Now `assert_matches!` expands to
```
match state {
ChildExit::KilledExternal => {}
ref left_val => {
$crate::panicking::assert_matches_failed(
left_val,
"ChildExit :: KilledExternal",
$crate::option::Option::None,
);
}
}
```
### Testing Instructions
No unused code warnings anymore.
👀
Closes TURBO-1894
---------
Co-authored-by: Chris Olszewski <Chris Olszewski>
