Test coverage for all plugins

As of this writing, the core impress.js API has ok test coverage, as does the navigation plugin. All other plugins were essentially implemented without tests. (Sorry, historical reasons.)

Now that they're merged to upstream impress.js though, we do have a goal of adding test coverage for all of them. Basically, it would be nice to have at least one `src/plugins/pluginname/pluginname_tests.js` file for each plugin.

This is a catch all ticket. If you want to help in creating tests for a plugin, please [file a new issue](https://github.com/impress/impress.js/issues/new) with a title like _"Add tests for plugin pluginname"_

Related: #657

This is a catch-all issue rather than a plugin-specific task. Start by choosing one plugin and inspecting its src/plugins/pluginname directory, using the existing core API and navigation plugin coverage as references. Done means the selected plugin has a src/plugins/pluginname/pluginname_tests.js file; file a separate issue for that plugin as requested.

Written by the indexing model from the issue text.