Disallow calling `#[test]` functions from non-`#[test]` code

Calling `#[test]` functions from non-test code shouldn't be particularly harmful, but is just a bit confusing and would make supporting `dev-dependencies` more complicated (or at least produce confusing results).

We should check function calls to ensure `#[test]` functions are never called from non-test code.

No files or tests are named. Start by locating the compiler checks for function calls and the handling of #[test] functions, then add coverage showing that non-test code cannot call them; done means those calls are rejected while calls from test code remain supported.

Written by the indexing model from the issue text.