As of https://github.com/status-im/nim-eth/pull/379 `nim-eth` defines a
couple static test cases for merkle proof verification.
Since the EF has defined a `is_valid_merkle_branch` function in the spec
we are no longer using the custom implementation from `nim-eth`, but the
tests were never ported to target the new implementation. This patch now
follows up on that and integrates those tests from `nim-eth`.