Tests against NSS head do not work anymore

See https://github.com/cloudflare/tls-tris/pull/66 for more details
Ideally interoperability tests for tls-tris should be done only against hardcoded commit in 3rd party software, which will improve stability and save time spent on debugging.

Actually better solution seems to be not to have interoperability tests and simply use same approach as in ``handshake_client_test.go`` and ``handshake_server_test.go`` (which is uses data from "testdata" dir). Probably it makes sense to add such improvement after TLSv1.3 spec is finalized