fix: first drop mutex guard, then assert

the assert while holding the guard poisons the mutex and that causes
drop to panic because of the poisoned mutex, by design; see last
commit. by shortening the scope of the mutex guard we should once again
see more clear build failures :)

tests/wantlist_and_cancellation.rs

@@ -38,11 +38,13 @@ where
 }
 
 async fn check_cid_subscriptions(ipfs: &Node, cid: &Cid, expected_count: usize) {
+    let subscription_count = {
         let subs = ipfs.get_subscriptions().lock().unwrap();
         if expected_count > 0 {
             assert_eq!(subs.len(), 1);
         }
-    let subscription_count = subs.get(&cid.clone().into()).map(|l| l.len());
+        subs.get(&cid.clone().into()).map(|l| l.len())
+    };
     // treat None as 0
     assert_eq!(subscription_count.unwrap_or(0), expected_count);
 }