Extracted the wp_insert_post/wp_update_post argument construction into a
private build_postarr() helper so the post_date/post_date_gmt derivation
from the feed timestamp can be exercised in isolation. Added smoke tests
that pin down the create-path behavior (feed timestamp becomes post_date)
and the fallback path (no feed date leaves post_date unset for WordPress
to fill in at import time). Added minimal get_date_from_gmt and
get_gmt_from_date stubs to the test bootstrap so the new tests run
without WordPress loaded.
Keith Solomon