test coverage

[keewis]

The test coverage is not ideal right now:

Name                                    Stmts   Miss  Cover
-----------------------------------------------------------
blackdoc/__init__.py                       10      5    50%
blackdoc/blacken.py                        34     22    35%
blackdoc/classification.py                 13      2    85%
blackdoc/formats/__init__.py               20     12    40%
blackdoc/formats/doctest.py                38      4    89%
blackdoc/formats/none.py                    9      0   100%
blackdoc/formats/register.py               13      1    92%
blackdoc/tests/__init__.py                  0      0   100%
blackdoc/tests/data.py                      4      0   100%
blackdoc/tests/test_blacken.py              6      0   100%
blackdoc/tests/test_classification.py      32      0   100%
blackdoc/tests/test_doctest.py             21      0   100%
blackdoc/tests/test_none.py                17      0   100%
-----------------------------------------------------------
TOTAL                                     217     46    79%

so let's increase it. Most of the needed tests are integration tests, though, which are a little bit harder to write.