h3-proxy: fix callback return values, and a typo in tests

- replace literal -1 with `NGHTTP3_ERR_CALLBACK_FAILURE` in nghttp3
  callback.
- replace `NGHTTP3_ERR_CALLBACK_FAILURE` with
  `NGTCP2_ERR_CALLBACK_FAILURE` in ngtcp2 callbacks.
- test_60_h3_proxy: fix non-critical typo in symbol.

Spotted by GitHub Code Quality

Follow-up to e78b1b3ecc #21153

Closes #21802

tests/http/test_60_h3_proxy.py

@@ -571,7 +571,7 @@ class TestH3ProxyUdpTunnel:
         self, env: Env, httpd, nghttpx, nghttpx_fwd
     ):
         _require_available(
-            httpd=httpd, nghttpx=nghttpx, nghttps_fwd=nghttpx_fwd
+            httpd=httpd, nghttpx=nghttpx, nghttpx_fwd=nghttpx_fwd
         )
         curl = CurlClient(env=env)
         url = f"https://localhost:{httpd.ports['https']}/data.json"