refactor dsl_test.rb into named sections
Extend `appcast` stanza to support multiple keys

- `:sha256`
- `:format` (default is `:sparkle`)
- `:latest_version` (experimental, may be removed)
tests for `appcast` stanza
Merge pull request #4847 from rolandwalker/extend_appcast_stanza

DSL: extend `appcast` stanza