Add new tests for abfe gather

[IAlibay]

With #1776 the structure of the gathered units has changed, so we have to access the setup_results to get the input chemical systems.

A fix for this was added in #1776 but the test files were not updated.

[atravitz]

@IAlibay to be clear - is this a request to update test data for the existing tests, or add new tests?

[IAlibay]

Update existing test data? Or maybe make a new test that uses the new test data?

[atravitz]

note: we will need to rerun the abfe simulations that created the original test data here - either hannah or mike should know.