text: Title: Move BrownCorpus from word2vec to gensim.corpora
  Body: Not a high-priority at all, but it'd be more sensible for such a tutorial/testing utility corpus to be implemented elsewhere - maybe under `/test/` or some other data- or doc- related module – rather than in `gensim.models.word2vec`.
  
  _Originally posted by @gojomo in https://github.com/RaRe-Technologies/gensim/pull/2939#discussion_r493820305_