Define and maybe provide Mock implementation/framework for contracts

[xgreenx]

During integration testing of the cross-contract execution can be useful to mock some contracts and verify that some logic has been invoked.
Let's define in this issue how it can be implemented in terms of customization and usability.

[h4x3rotab]

I got some raw ideas for the mocking the cross contract calls, but may need more polish: use-ink/ink#788 (comment)

[h4x3rotab]

Reference implementation: #136