tlv+watchtower: move bench test

The benchmark tests import both the tlv and watchtower packages. To make
it possible to extract the tlv package into its own submodule, this test
is better located in the watchtower package.

watchtower/tlv_bench_test.go

@@ -1,4 +1,4 @@
-package tlv_test
+package watchtower_test
 
 import (
 	"bytes"