[Coverage] output-tests/ is missing structure/ subdirectory documented in README

The output-tests/README.md explicitly describes two categories of output tests , content and structure, and states:

"Structure tests verify that the structures of the various formats (i.e. flag, basic, detailed, verbose for 2019-09/2020-12…) are correct."

However, output-tests/draft2020-12/ and output-tests/draft2019-09/ only contain a content/ subdirectory. The structure/ subdirectory is entirely absent for both drafts. The README even contains a TODO comment acknowledging the gap:

<!-- This general.json file may be added to the structure tests later, but I haven't gotten to them yet, so I don't know. -->

This means there is currently no test coverage verifying that implementations produce structurally correct output for any of the four formats (flag, basic, detailed, verbose). Any implementation could emit a malformed output structure and the suite would not catch it.

Expected: output-tests/draft2020-12/structure/ and output-tests/draft2019-09/structure/ each contain test files for flag, basic, detailed, and verbose output formats.

Actual: Neither structure/ directory exists.

Start with output-tests/README.md and compare the existing content/ directories under output-tests/draft2020-12/ and output-tests/draft2019-09/. Follow the documented output-test layout to add structure/ test files for flag, basic, detailed, and verbose in both drafts. Done means both structure/ directories exist and cover all four formats.