Fix graphviz_test with test table

The tests were explicitly checking order where the underlying implementation is `map[string]*BasicNode`. This PR addresses the order which is not known and within the test I create a test table to allow multiple tests (2 in this PR rather than 1).


Note: Similar to Pull #61, _however_ I am only addressing the graphviz test by introducing a test table (for  multiple tests), using deep equals, and adding additional tests.

Would appreciate any comments.