Integration tests for non-CSV export formats

All the blackbox integration tests currently rely on writing to CSV. This prevents, e.g., writing a regression test for #613. We need a way to specify which format(s) we expect and generalise the test harness to handle that.

Start by locating the blackbox integration-test harness and tracing where it assumes CSV output; review issue #613 as the motivating regression case. Done means the harness can declare one or more expected export formats and integration tests cover non-CSV output without breaking existing CSV tests.

Written by the indexing model from the issue text.