JavaScript reorganization in core broke tests

The [JavaScript reorganization and build step recently committed to core](https://core.trac.wordpress.org/ticket/43055) removed some third-party JS files from core's `src/` directory that Fieldmanager links to [in `tests/js/index.html`](https://github.com/alleyinteractive/wordpress-fieldmanager/blob/10080e822affeea0a71ec717746e153f9a4406ec/tests/js/index.html#L24-L32). The tests now fail because the test dependencies are gone.

I'm not sure of a straightforward way to access to Fieldmanager's test dependencies now except for linking to the built version of WordPress at core.svn.wordpress.org instead of the `src` files at develop.svn.wordpress.org.