This provides more flexibility in repository setup while remaining backwards
compatible.
The changes to test_reports_query are just to accommodate the new
transaction in 2020-expenses.beancount.
Building a string and loading it means Beancount can never cache any
load. It only caches top-level file loads because options in the
top-level file can change the semantics of included entries.
Instead use load_file as much as possible, and filter entries as
needed.