Use a better design to better control the texinfo generation.

* bzrlib/tests/doc_generate/builders/test_texinfo.py:
(TestTextGeneration.test_block_quote): Add a test that were missed
because the previous design was relying on side-effects.
(TestTableGeneration.test_table): Note that sphinx seems to have a
bug here.
(TestTocTreeGeneration.test_toctree): The syntax was wrong.

* bzrlib/doc_generate/writers/texinfo.py:
(TexinfoTranslator): Change the design by letting nodes collect
the data and processed them as they see fit instead of relying on
translator variables (which doesn't scale).