fix the Trie tests in runtest.py

This fixes the `trie.Trie` tests in `runtest.py` by
- adapting the code to the new test data structure and
- catching exceptions raised for root nodes with a single child (a 2-element list) and calling `encode('hex')` on their string representation.

This patch fixes issues https://github.com/ethereum/pyethereum/issues/7 and https://github.com/ethereum/pyethereum/issues/10