Fix unit test

If running test behide a proxy, we may get the error code
403 Forbidden which will fail line 135 for the last testcase.

Detail:
```
metrics not as expected: notifications.EndpointMetrics{Pending:0,
Events:0, Successes:4, Failures:0, Errors:0,
Statuses:map[string]int{"307 Temporary Redirect":0, "400 Bad Request":0,
"403 Forbidden":0, "200 OK":4}} !=
notifications.EndpointMetrics{Pending:0, Events:0, Successes:4,
Failures:0, Errors:0, Statuses:map[string]int{"400 Bad Request":0, "200
OK":4, "307 Temporary Redirect":0}}
```

Immediate close will fix that

Signed-off-by: Hu Keping <hukeping@huawei.com>
Signed-off-by: Derek McGowan <derek@mcgstyle.net>

notifications/http_test.go

@@ -5,6 +5,7 @@ import (
 	"encoding/json"
 	"fmt"
 	"mime"
+	"net"
 	"net/http"
 	"net/http/httptest"
 	"reflect"
@@ -94,6 +95,21 @@ func TestHTTPSink(t *testing.T) {
 	var expectedMetrics EndpointMetrics
 	expectedMetrics.Statuses = make(map[string]int)
 
+	closeL, err := net.Listen("tcp", "localhost:0")
+	if err != nil {
+		t.Fatalf("unexpected error creating listener: %v", err)
+	}
+	defer closeL.Close()
+	go func() {
+		for {
+			c, err := closeL.Accept()
+			if err != nil {
+				return
+			}
+			c.Close()
+		}
+	}()
+
 	for _, tc := range []struct {
 		events     []Event // events to send
 		url        string
@@ -121,8 +137,8 @@ func TestHTTPSink(t *testing.T) {
 			failure:    true,
 		},
 		{
-			// Case where connection never goes through.
+			// Case where connection is immediately closed
-			url:     "http://shoudlntresolve/",
+			url:     closeL.Addr().String(),
 			failure: true,
 		},
 	} {