[feature] Add lightweight authorization test helpers

## Background

Testing authorization behavior with Casbin often results in repetitive `Enforce` assertions with limited failure context.

### Proposal

Introduce a small set of authorization test helper functions as an external package.

Example:

```go
authztest.AssertAllow(t, e, "alice", "data1", "read")
authztest.AssertDeny(t, e, "bob", "data1", "write")
```

### Design Notes

* Helpers are thin wrappers around `Enforce`