test/stream: fix -Woverloaded-virtual warning

Explicilty override `uvw::uv_type<U>::init()` in `fake_stream_handle`
to hide the following compiler warning:

```
src/uvw/uv_type.hpp:32:17: warning: ‘int uvw::uv_type<U>::init() [with U = fake_stream_t]’ was hidden [-Woverloaded-virtual]
   32 |     virtual int init() {
      |                 ^~~~
test/uvw/stream.cpp:12:9: note:   by ‘fake_stream_handle::init(Args&& ...)’
   12 |     int init(Args &&...) override {
                ^~~~
```

test/uvw/stream.cpp

@@ -12,4 +12,8 @@ struct fake_stream_handle: uvw::stream_handle<fake_stream_handle, fake_stream_t>
     int init(Args &&...) {
         return 0;
     }
+
+    virtual int init() override {
+        return 0;
+    }
 };