test: fix double close channel bug by removing wait in lightningNode.stop()

This commit fixes a panic bug caused by two calls to a process’
cmd.Wait() method. If two nodes incurred a fatal error as soon as they
were created, then both the goroutine detected to recovering the stderr
data and the defer statement would case a double channel close due to
the simultaneous calls to Wait().

networktest.go

@@ -260,8 +260,6 @@ func (l *lightningNode) stop() error {
 		return nil
 	}
 
-	defer l.cmd.Wait()
-
 	if runtime.GOOS == "windows" {
 		return l.cmd.Process.Signal(os.Kill)
 	}