Abstraction of testing layer

[Xenomega]

The project aims to have first party support for some testing strategies (property tests, assertion-based testing, etc). However, on an API level we should provide hooks such as pre-tx-execution and post-tx-execution that allow people to use the Fuzzer with their own hooks to report their own custom test results. Our own testing strategies should be implemented using the generic test case provider interface that others can also use, to remain as agnostic as we can and ensure the API supports all the operations one might expect.

This PR is dependent on #21 , which will introduce a generic test result interface to be used in this PR.