tests: handle ECONNREFUSED in uds_stream::epollhup (#6778)

## Motivation

Currently, the test `uds_stream::epollhup` expects that a
`UdsStream::connect` future to a Unix socket which is closed by the
accept side to always fail with `io::ErrorKind::ConnectionReset`. On
illumos, and potentially other systems, it instead fails with
`io::ErrorKind::ConnectionRefused`.

This was discovered whilst adding an illumos CI job in PR #6769. See:
#6769 (comment)

## Solution

This commit changes the test to accept either `ConenctionReset` or
`ConnectionRefused`. This way, we are more tolerant of different
operating systems which may decide to return slightly different errnos
here. Both ECONNREFUSED and ECONNRESET seem reasonable to expect in this
situation, although arguably, ECONNREFUSED is actually more correct: the
acceptor did not accept the connection at all, which seems like
"refusing" it to me...

tokio/tests/uds_stream.rs

@@ -406,6 +406,17 @@ async fn epollhup() -> io::Result<()> {
     drop(listener);
 
     let err = connect.await.unwrap_err();
-    assert_eq!(err.kind(), io::ErrorKind::ConnectionReset);
+    let errno = err.kind();
+    assert!(
+        // As far as I can tell, whether we see ECONNREFUSED or ECONNRESET here
+        // seems relatively inconsistent, at least on non-Linux operating
+        // systems. The difference in meaning between these errnos is not
+        // particularly well-defined, so let's just accept either.
+        matches!(
+            errno,
+            io::ErrorKind::ConnectionRefused | io::ErrorKind::ConnectionReset
+        ),
+        "unexpected error kind: {errno:?} (expected ConnectionRefused or ConnectionReset)"
+    );
     Ok(())
 }