Only perform search / replace on tables registered to `$wpdb`

`$wpdb->prepare( "SHOW TABLES LIKE %s", like_escape( $prefix ) . '%'` is lossy,
as it matches a separate WordPress install with similar prefix (e.g. `wp_` and `wp_en_`)
Explicit test to make sure we aren't clobbering unexpected tables
Merge pull request #1304 from wp-cli/tables-1304

`search-replace` should only apply to install's tables