Use t.Cleanup() rather than defer to shutdown test servers

Thanks @phillebaba!

src/pkg/packager/sources/new_test.go

@@ -130,7 +130,7 @@ func TestPackageSource(t *testing.T) {
 		defer f.Close()
 		io.Copy(rw, f)
 	}))
-	defer ts.Close()
+	t.Cleanup(func() { ts.Close() })
 
 	tests := []struct {
 		name   string

src/pkg/utils/network_test.go

@@ -85,7 +85,7 @@ func TestDownloadToFile(t *testing.T) {
 		}
 		rw.Write([]byte(content))
 	}))
-	defer srv.Close()
+	t.Cleanup(func() { srv.Close() })
 
 	tests := []struct {
 		name        string