add simple test cases with exhaustive enumeration of all possible combos of node times

[awohns]

@hyanwong has worked out a method for exhaustively enumerating all possible combinations of node times. This allows calculation of the exact likelihood of a particular set of node times given a tree topology: i.e. the absolutely correct answer even if there are loops in a tree sequence (will provide a ground truth for solving #165).

We should create tests/test_exahustive.py for these types of tests.