Unify tests

[j-a-m-l]

Currently we don't have a unified way to test all the packages: some of them are using core-test-utils or have received more love than others.

• Refactor to create more custom matchers
• Refactor to remove duplicated fixtures (genesisBlock.json, etc.)

[j-a-m-l]

I've updated this issue to defined several specific tasks. Suggestions are welcomed.

[faustbrian]

Any progress on this? Has already been a month.

[j-a-m-l]

I've been working on #783 to fix some problems that affect almost all of the packages that require setting up a blockchain. When that it's finished, I will continue with these points.

[faustbrian]

@air1one will take over all testing related tasks on core now.

[air1one]

I think we can close this as I did a first good refactoring across the packages, using core-test-utils when useful. (also refactored core-test-utils itself).

Also wrote a doc to explain how our tests are structured and how to add / update tests for anyone to be able to do so : see here ArkEcosystem/docs#52 (PR being reviewed)