[Feature]: Allow setting msg::sender in unit tests

[alexfertel]

No description provided.

[alexfertel]

A spike trying to get this to work resulted in these changes. In the end, it is impossible to set msg::sender right now since it is a global CachedOption private to the SDK, so we can't access it.

[0xNeshi]

Maybe a wrapper around msg::sender similar to Solidity's Context would alleviate the issue?
If there were a pub trait IContext that was implemented for the Context contract (that other contracts inherit), then the trait can be mocked in tests (e.g. using mockall)