Make javatests/flows package have public visibility

It's reasonable for people extending the code base to want access to
ResourceFlowTestCase and others.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=141765523

javatests/google/registry/flows/BUILD

@@ -1,6 +1,6 @@
 package(
     default_testonly = 1,
-    default_visibility = ["//java/google/registry:registry_project"],
+    default_visibility = ["//visibility:public"],
 )
 
 licenses(["notice"])  # Apache 2.0