Upgrade official Ethash tests / Remove ethereumjs-testing dependency

[holgerd77]

Tests should be upgraded to the latest test state from the ethereum/tests repository (not sure if necessary though since tests are not updated there for quite some time as well).

Along the ethereumjs-testing dependency should be removed analogue to e.g. ethereumjs/ethereumjs-block#61, since the dependency is super-heavy (several 100 MBs) and the tests rarely change.

This would be a pre-requisite for #20 (the monorepo part), since we don't want to draw in this dependency within the monorepo.