Don't build tests for plugins that weren't build

We should no longer depend on the various HAVE_* variables exclusively,
but also on the user options which may select them.

Change-Id: I896eb8bae65a3f80c32698eb586dd3a3a1ede492
Reviewed-on: http://review.couchbase.org/43453
Reviewed-by: Brett Lawson <brett19@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>