fix(complement): Build with the openreg feature

Since challengeless registration was gated behind the `openreg` feature,
building with plain `cargo build -p conduwuit` produces a server that
refuses Complement's registrations, so almost every test fails.

This commit updates the testing docs and the runner's build hints to
enable the feature.

docs/development/testing.mdx

@@ -12,11 +12,14 @@ Install Git, Rust, Docker, Go, and `jq`. From this repository's root, build the
 server image and run the suite:
 
 ```bash
-cargo build -p conduwuit
+cargo build -p conduwuit --features openreg
 docker build -t continuwuity:complement -f docker/complement.Dockerfile .
 ./bin/complement
 ```
 
+The `openreg` feature is required: without it the server refuses the open
+registrations Complement uses to create test users, and almost every test fails.
+
 The run can take up to an hour. The runner writes the full Go test stream to
 `tests/test_results/complement/test_logs.jsonl` and a sorted pass/fail/skip
 summary to `tests/test_results/complement/test_results.jsonl`. It exits nonzero