Fix failing test `TestGlobWithMultipleFiles` in pkg/promtail/targets (#466)

TestClient used for watching log file events appends logs to a list
Due to race condition some of the logs added to it goes missing
Adding a Mutex to it to lock messages before adding logs to it

pkg/promtail/targets/filetarget_test.go

@@ -7,6 +7,7 @@ import (
 	"os"
 	"path/filepath"
 	"sort"
+	"sync"
 	"testing"
 	"time"
 
@@ -724,11 +725,15 @@ func TestMissing(t *testing.T) {
 type TestClient struct {
 	log      log.Logger
 	messages []string
+	sync.Mutex
 }
 
 func (c *TestClient) Handle(ls model.LabelSet, t time.Time, s string) error {
-	c.messages = append(c.messages, s)
 	level.Debug(c.log).Log("msg", "received log", "log", s)
+
+	c.Lock()
+	defer c.Unlock()
+	c.messages = append(c.messages, s)
 	return nil
 }