In a test environment, rely on the loading of
`:active_support_test_case`.

Introduce the `:active_record_fixture_set` hook for the Active Storage
engine to listen for during the load process in a development
environment (like when running `db:fixtures:load`).

Since this commit moves the task-aware path resolution out of the block
that provided local variables, it recreates part of the fixture
directory path resolution logic.