logtail: start a local server for TestFastShutdown

Right now TestFastShutdown tries to upload logs to localhost:1234,
which will most likely respond with an error. However if one has an
actual service running on port 1234, it would receive a connection
attempting to POST every time the unit test runs.

Start a local server and direct the upload there instead.

Signed-off-by: Denton Gentry <dgentry@tailscale.com>

logtail/logtail_test.go

@@ -16,8 +16,12 @@ func TestFastShutdown(t *testing.T) {
 	ctx, cancel := context.WithCancel(context.Background())
 	cancel()
 
+	testServ := httptest.NewServer(http.HandlerFunc(
+		func(w http.ResponseWriter, r *http.Request) {}))
+	defer testServ.Close()
+
 	l := NewLogger(Config{
-		BaseURL: "http://localhost:1234",
+		BaseURL: testServ.URL,
 	}, t.Logf)
 	l.Shutdown(ctx)
 }
@@ -30,6 +34,7 @@ func TestUploadMessages(t *testing.T) {
 			w.Header().Set("Content-Type", "application/json; charset=utf-8")
 			uploads += 1
 		}))
+	defer testServ.Close()
 
 	l := NewLogger(Config{BaseURL: testServ.URL}, t.Logf)
 	for i := 1; i < 10; i++ {