[client] Keep the peer endpoint open in the sweep test

connPair closed the accepted connection right after the handshake, so the
reads in TestSweepClosesRegisteredConns failed on the peer's own close
rather than on the sweep. The test passed even with Sweep's close loop
removed. Hold the peer until cleanup so the read errors come from Sweep.

client/netsweep/netsweep_test.go

@@ -93,7 +93,9 @@ func TestNilSweeperIsNoop(t *testing.T) {
 	assert.NoError(t, ctx.Err(), "nil sweeper must not cancel the dial context")
 }
 
-// connPair dials a loopback TCP connection against a throwaway listener.
+// connPair dials a loopback TCP connection and keeps the accepted peer open
+// until the test ends: a peer that closed early would make the connection
+// unreadable on its own, so a read error after the sweep would prove nothing.
 func connPair(t *testing.T) net.Conn {
 	t.Helper()
 
@@ -105,15 +107,26 @@ func connPair(t *testing.T) net.Conn {
 		}
 	})
 
+	accepted := make(chan net.Conn, 1)
 	go func() {
 		conn, err := l.Accept()
 		if err != nil {
+			close(accepted)
 			return
 		}
-		_ = conn.Close()
+		accepted <- conn
 	}()
 
 	conn, err := net.Dial("tcp", l.Addr().String())
 	require.NoError(t, err)
+
+	peer, ok := <-accepted
+	require.True(t, ok, "listener must accept the dialed connection")
+	t.Cleanup(func() {
+		if err := peer.Close(); err != nil {
+			t.Logf("peer close error: %v", err)
+		}
+	})
+
 	return conn
 }