So far, the `SlidingSync.pos` field was public to the crate. In order to avoid
breaking the internal state of this type, its visibility is now private.
However, we need to be able to change the value when testing the
`SlidingSync` type itself. To achieve that, this patch removes the old
`force_sliding_sync_pos` function, and implements 2 new functions: `set_pos`
and `pos` directly on `SlidingSync` only when `#[cfg(any(test, feature ="testing"))]`.
Ivan Enderlin
5727726e5d