Renamed some tests, introducing subsidiary directories
for fb2, docx, epub.
Cleaned up tests in cabal file.
Combined dokuwiki-writer and dokuwiki_inline_formatting tests.
Renamed epub test files so they're identified more clearly as
epub: features.{epub,native} -> epub.features.{epub,native},
and similarly with formatting.{epub,native}.
Added epub test files to cabal file, so they'll be included in
the tarball.
