Make TestError an effectively-never instantiated struct

[wiktor-k]

One of clients of this library return errors from their test functions and explicitly into() them. Rust nightly started issuing lint failures for TestError being uninhabited (and as such the code being unreachable):

note: this expression has type `testresult::TestError`, which is uninhabited
   --> nethsm/src/user.rs:804:31
    |
804 |             Err(error) => Err(format!("Expected to fail with a Error::InvalidUserIds but got a different error instead:\n{error}").into()),
    |                               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

warning: unreachable expression
   --> nethsm/src/user.rs:805:27
    |
805 |             Ok(user_id) =>Err(format!("Expected to fail with a Error::InvalidUserIds but succeeded instead:\n{user_id}").into())
    |                           ^^^^-------------------------------------------------------------------------------------------------^
    |                           |   |
    |                           |   any code following this expression is unreachable
    |                           unreachable expression

This PR converts the TestError to a struct that is effectively never instantiated. Additionally it marks it as #[non_exhaustive] so that it is not possible to instantiate an object of this type outside of our crate.

CCing @swallez @ijackson in case you have better ideas 😅

Edit: it seems the beta (which currently complains) will become stable on December 11th so we'd need that merged and a new version released before that date.

[wiktor-k]

At Signstar we decided to rewrite the test code instead of relaxing testresult.

So, closing this PR as not-planned but it may be useful as a reference for others.