internal: rename package for test alone file (#3855)

Otherwise glaze complains.
diff --git a/xds/internal/client/tests/client_test.go b/xds/internal/client/tests/client_test.go
index 3c96175..7887835 100644
--- a/xds/internal/client/tests/client_test.go
+++ b/xds/internal/client/tests/client_test.go
@@ -16,7 +16,7 @@
  *
  */
 
-package tests
+package tests_test
 
 import (
 	"testing"