Refactor tests.

* bzrlib/tests/doc_generate/builders/test_texinfo.py:
(TestSphinx.build, TestSphinx.create_content)
(TestSphinx.assertContent): Some helpers to reduce duplication and
better document the intent.

* bzrlib/doc_generate/writers/texinfo.py:
(TexinfoTranslator.depart_block_quote): Make it paragraph-like
with a final newline.
(TexinfoTranslator.depart_tgroup): The table is now followed by a
new line.