Tests: upstream read timeout tests.

[climagabriel]

Tests for climagabriel/nginx PR: data already read from an upstream is flushed to the client on a read timeout. The four new timeout assertions fail on nginx 1.31.5 and pass with that PR; proxy_noclose.t and proxy_upgrade.t pass too.

• proxy_timeout.t covers a fixed-length body received with the headers, a fixed-length body received later, a cache-enabled location, and a response without Content-Length. The four assertions are TODO before nginx 1.31.6; update the gate if the change misses that release. It also verifies that the incomplete response is not cached and that a timed-out chunked response has no terminating zero-length chunk.
• proxy_noclose.t: the existing TODO on "bad backend - no content length" is gated on 1.31.6, mirroring freenginx nginx-tests 718f3b83e5.

When running as root, set TEST_NGINX_GLOBALS='user root;'; otherwise the cache location returns 500 because the worker cannot open the cache path.

https://claude.ai/code/session_01BERKTkJd6r3FPviAuD9SyE