You can use Deploy to Bitrise.io step to achieve that. More info in devcenter: http://devcenter.bitrise.io/tips-and-tricks/attach-any-file-to-build/#deploy-a-directory
Archive with test reports will be available for download on Apps & Artifacts tab.
Online preview (without downloading, for results other than single files) is not yet supported Out Of The Box however, there are some workarounds: Artifacts: Make it possible to view HTML files with relative links (CSS etc. refs)