Improved tests for wp-admin/includes/plugin.php.

- Improved cleanup for existing tests, by ensuring that plugins are deactivated.
- New tests for `get_plugin_files()`, `get_mu_plugins()`, `_sort_uname_callback()`, `get_dropins()`, `is_network_only_plugin()`, `activate_plugins()`, `validate_active_plugins()`, and `is_uninstallable_plugin()`.

Props sgrant.
See #30860.

git-svn-id: http://develop.svn.wordpress.org/trunk@31002 602fd350-edb4-49c9-b593-d223f7449a82