rpcwallet: fix RPC wallet health check connection leak

Fixes #6329.
This commit fixes a connection leak in the RPC wallet's health check. By
not closing the test connection the watch-only node would slowly stack
up connections and eventually hit the ulimit.

lnwallet/rpcwallet/healthcheck.go

@@ -11,7 +11,7 @@ import (
 // configuration.
 func HealthCheck(cfg *lncfg.RemoteSigner, timeout time.Duration) func() error {
 	return func() error {
-		_, err := connectRPC(
+		conn, err := connectRPC(
 			cfg.RPCHost, cfg.TLSCertPath, cfg.MacaroonPath, timeout,
 		)
 		if err != nil {
@@ -19,6 +19,15 @@ func HealthCheck(cfg *lncfg.RemoteSigner, timeout time.Duration) func() error {
 				"signing node through RPC: %v", err)
 		}
 
+		defer func() {
+			err = conn.Close()
+			if err != nil {
+				log.Warnf("Failed to close health check "+
+					"connection to remote signing node: %v",
+					err)
+			}
+		}()
+
 		return nil
 	}
 }