Cleanup routingtable

The point of calling `super(TestOptimizedTreeRoutingTable, self).test_init()` is exactly to reuse the `test_init` of the superclass in order to test that `OptimizedTreeRoutingTable` conforms to the superclass API. Thus, these tests need not be duplicated here.

The whole point is to reuse test code, but maybe I should write a couple of docstrings so that it's clear.