Adds a 'Reset cache' button to the Podcast RSS Import settings page that
clears the SimplePie feed-cache rows (both the cached XML and the
modification timestamp) plus the importer's per-feed import lock for the
configured feed. Multisite site-transient variants are also cleared.
Use this when the importer is skipping items or refusing to re-fetch a
feed that has changed at the source. The next 'Import now' click will
re-pull the XML from the network.
clear_feed_cache() is exposed as a public method so it is unit-testable,
and a do_action hook (rss2cpt_cache_cleared) lets other code react when
the cache is cleared. Two smoke tests pin down the expected option and
site-transient names, and a third ensures an empty feed URL is a no-op.
Keith Solomon
7887f7d9ce