Add a step for scenarios that need a specific set of themes #376

Several feature files start every scenario with wp theme delete --all --force followed by one or more wp theme install calls, to get a known set of themes. Examples are the theme auto-updates features and theme.feature in extension-command, and menu-location.feature in entity-command.

Deleting all themes takes about 2 seconds and each install about 3 seconds, so these Backgrounds add 5 to 12 seconds to each of 54 scenarios per suite run.

A step that leaves exactly the listed themes installed would express the intent of these Backgrounds directly and avoid most of that time.