Clean up meaningless "post body" language

A post is the chunk of wiki text, it doesn't have a separate body (see
https://www.mediawiki.org/wiki/Flow_Portal/Nomenclature ).  So this
rephrases some test language.  Also fix a mistaken "saved topic body"
reference.

There's still mention of "topic body" and "create a Body of Flow Topic
into Flow body".  When you create a topic you're invited to add a first
post to it at the same time. FWIW I think the only reasonable meaning of
"topic body" is the tree of posts under a topic that is hidden when you
click a topic bar

Change-Id: I7bcf5bccf98477534ad53dd0f084fb1539db9710