Tests: rcvbuf and sndbuf directives for upstream sockets

[patrikwl]

Adds stream_proxy_socket_buf.t covering proxy_socket_rcvbuf
and proxy_socket_sndbuf in the four configurable forms: off,
explicit size, max, and default-no-directive. Each subtest
asserts a small message proxies through end-to-end, exercising
the setsockopt() path during outbound connect.

The directives are introduced in nginx/nginx#1298.

Refs nginx/nginx#1297.

[patrikwl]

Update: I’ve extended this tests PR to cover the HTTP upstream follow-up as well.

The PR now includes:

• stream_proxy_socket_buf.t

  • still covers proxy_socket_rcvbuf and proxy_socket_sndbuf
  • covers off, explicit size, max, and default-no-directive
  • now also verifies the configured upstream socket buffers with ss -tnpemi

• proxy_socket_buf.t

  • added for HTTP upstream socket buffer directives
  • covers proxy_socket_rcvbuf and proxy_socket_sndbuf at runtime
  • verifies the kernel-visible upstream socket buffers with ss -tnpemi
  • covers config parsing for:
    • fastcgi_socket_rcvbuf / fastcgi_socket_sndbuf
    • grpc_socket_rcvbuf / grpc_socket_sndbuf
    • memcached_socket_rcvbuf / memcached_socket_sndbuf
    • scgi_socket_rcvbuf / scgi_socket_sndbuf
    • uwsgi_socket_rcvbuf / uwsgi_socket_sndbuf
  • covers invalid values and duplicate directive handling

Verified locally with:

```sh
prove -v proxy_socket_buf.t stream_proxy_socket_buf.t
```

Result: all tests pass.

The corresponding nginx source changes are in nginx/nginx#1298.

Refs nginx/nginx#1297.