Move from etap to eunit

Pursuant to an IRC/Twitter discussion, it seems that there is some
support (pjdavis1970, janl) to move off of etap. Motivations include etap being unmaintained for 2+ years, eunit shipping with Erlang, and least importantly etap tests being completely broken on the Windows build.

There are only 63 .t files in the entire source tree; hopefully this is work that can be weekend-tackled by one person, or could be tag-teamed with little friction.

There was a subsequent discussion about the current, existing JS tests; while there are issues with these and room for improvement, this bug is strictly about our white-box etap unit tests and larger multi-instance tests that may not best be accomplished by a JS framework.