message: add build-from-source info to Tests architecture docs
  it's mentioned in the README, but it's worth mentioning again in the
  Tests section of the architecture doc.
diff: @@ -103,6 +103,9 @@ assert_that(
  );
  ```
  +Alternatively to build and run a custom version of cargo simply run `cargo build`
  +and execute `target/debug/cargo`.
  +
  ## Logging
  Cargo uses [`env_logger`](https://docs.rs/env_logger/*/env_logger/), so you can set
  