Better model of sudo test in examples.

[jawoznia]

          Is there any test case asserting that when we call sudo_custom, the Sudo message is generated and sent under the hood? I can imagine a situation where `#[msg(sudo)]` and `sudo_custom()` might, by mistake, operate on an Exec or Query message.

Originally posted by @kulikthebird in #301 (comment)

While testing the exec message we model the flow as:
multitest(sends message) -> contract (receives message and return custom message) -> custom module (receives and parses custom message)

In case of sudo the flow should be reversed
multitest(sends message) -> custom module -> contract (receives message and return custom message)
custom module (receives and parses custom message)

Discuss with @DariuszDepta how such flow should be modeled in multitest.