Alter behaviour of flycheck-rust-check-tests

- stop passing `--test` to `rustc` in favour of simply passing `--cfg test`
- the former approach causes `rustc` to add a test harness, effectively
  removing any existing `main()`, meaning `main()` is not checked for errors.
- this approach also means that such test code can live happily with crates
  using `#![nostd]` as the test harness is no longer introduced.