Consider refactoring BNF coding systems import data to be like other tests

[StevenMaude]

As mentioned in #2264, test_import_data.py for the BNF coding system currently uses a pytest fixture for the data to be imported.

Other analogous tests for other coding systems use repository data fixtures instead; example.

We could make these tests slightly more uniform by making the BNF import data tests do the same: use a repository data fixture, instead of a generated-by-pytest fixture.

On the other hand, we may later refactor these code and tests entirely, and it may not be worth doing.