Add unittest coverage

[pcostell]

Right now, many of the tests are essentially unittests but are written using the doctests. If these were made unittests it would be easier to add additional coverage.

[mgedmin]

I've no objections. I mostly wrote doctests because I believed at the time that they lead to more readable tests. They have some downsides: it's harder to share test setup code between doctests.

BTW for coverage purposes it's much easier to write isolated doctests like https://github.com/mgedmin/objgraph/blob/master/tests.py#L218-L228 than to try to extend one of the txt files in https://github.com/mgedmin/objgraph/tree/master/docs. Adding unittest.TestCase-based unit tests to tests.py would be more or less equivalent to adding a new doctest_xxx function, IMHO.

[mgedmin]

This is fixed, I think?