Fix failing tests

The pipeline client used in some tests could not have its Close method
called more than once, given the changes introduces in the previous
commits, some test scenarios called it more than once.

This is fixed by using a sync.Once in ChanClient.Close.

libbeat/publisher/testing/testing.go

@@ -19,6 +19,8 @@ package testing
 
 // ChanClient implements Client interface, forwarding published events to some
 import (
+	"sync"
+
 	"github.com/elastic/beats/v7/libbeat/beat"
 )
 
@@ -31,6 +33,7 @@ type ChanClient struct {
 	done            chan struct{}
 	Channel         chan beat.Event
 	publishCallback func(event beat.Event)
+	closeOnce       sync.Once
 }
 
 func PublisherWithClient(client beat.Client) beat.Pipeline {
@@ -68,7 +71,9 @@ func NewChanClientWith(ch chan beat.Event) *ChanClient {
 }
 
 func (c *ChanClient) Close() error {
-	close(c.done)
+	c.closeOnce.Do(func() {
+		close(c.done)
+	})
 	return nil
 }