For instructions on how to use the Theme Test Data please see https://codex.wordpress.org/Theme_Unit_Test
To download the Accessibility teams test file, please go to: https://github.com/wpaccessibility/a11y-theme-unit-test
Note that issues and pull requests for the theme preview file will not be reflected on WordPress.org. This file is for testing only.
If you find a problem with the theme unit test: go here https://github.com/WPTT/theme-test-data/issues and see if an issue has been opened for this problem. If not, open an issue.
To help solve an issue: fork the repository, create a branch for the update, and make a pull request. Make sure that you check the box allowing others to make edits.
To review a pull request: view it and/or test it locally and add your comments.
If a pull request is ready to be merged and this has been noted already in a review but you do not have permission to merge, then be patient. 🙂 If the pull request that is ready to be merged has not been merged for some time, ping @pattonwebz on the issue.
After merging, go to this codex page, https://codex.wordpress.org/Theme_Unit_Test, log in, and update the last updated date.