itest: avoid blocking on failed litd exits

Close the process exit signal before forwarding litd startup errors from
the wait goroutine. This lets the harness observe that the process has
already exited even when no receiver is currently ready on the error
channel.

Use a non-blocking send for the captured process error so failed startup
paths do not hang the goroutine while holding back log finalization or
process exit handling.

This will be needed for the upcoming commit which adds itest coverage
of deprecated kvdb databases.

itest/litd_node.go

@@ -792,15 +792,21 @@ func (hn *HarnessNode) Start(litdBinary string,
 		defer hn.wg.Done()
 
 		err := hn.cmd.Wait()
-		if err != nil {
-			litdError <- fmt.Errorf("%v\n%v\n", err, errb.String())
-		}
-
 		// Signal any onlookers that this process has exited.
 		close(hn.processExit)
 
 		// Make sure log file is closed and renamed if necessary.
 		finalizeLogfile()
+
+		if err != nil {
+			select {
+			case litdError <- fmt.Errorf(
+				"%v\n%v\n", err, errb.String(),
+			):
+
+			default:
+			}
+		}
 	}()
 
 	// We may want to skip waiting for the node to come up (eg. the node