Allow unwrap() in tests when clippy::unwrap_used is denied (#11616)

## Problem

The proxy denies using `unwrap()`s in regular code, but we want to use
it in test code
and so have to allow it for each test block.

## Summary of changes

Set `allow-unwrap-in-tests = true` in clippy.toml and remove all
exceptions.

proxy/src/serverless/websocket.rs

@@ -177,7 +177,6 @@ pub(crate) async fn serve_websocket(
 }
 
 #[cfg(test)]
-#[expect(clippy::unwrap_used)]
 mod tests {
     use std::pin::pin;