Add expectation support

To be able to implement/provide mocking-like functionality, we need a way to test whether expected behavior has occurred.
The conventional way of doing this is with an `Expectation`-like object.

This task is to track adding expectations in some manner as a new module in `neotest`.

Start by locating the existing neotest modules and understanding how test behavior is currently represented. Define the scope and API for a new Expectation-like module, including how expected behavior is checked; the work is done when expectations support the intended mocking-like functionality and are covered by tests.

Written by the indexing model from the issue text.