Detect 8GB RAM and add GENSIM_BIG=1 for gensim/test/test_big.py
to the debian/rules and debian/tests/unittests files.

Fix gathering tests for test code that tries to load missing test files:

gensim/test/test_doc2vec.py
gensim/test/test_fasttext.py
gensim/test/test_word2vec.py
