Add integration tests and fixtures for `video` figure type

[cbutcosk]

Currently the integration tests use the HEAD commit's CLI to create a new publication with the default starter and build the site, PDF, and epub. Because the default starter does not contain a video tag these figures are not available for integration testing.

This issue is to add a _fixtures directory to the root _tests directory with YAML data hunks for figures.yaml and pages for using the YAML data, add a step to the existing integration tests that adds these figures and their pages to the test publication before building, and add an integration test for the publication-hosted video tag created by the fixture (see if checking the video tag's readyState is sufficient).