Fix `IssueCopyCommandTest` logic about the target adapter

[shulard]

Hello,

As discussed in the #546 PR, there is a problem with the destination adapter test logic.

Indeed, in the unit test, we start with the idea that the issue tracker to test is the one linked to the Application. With the recent updates it's not the case anymore. If the copy destination is not the same as source, we must create an adapter.

I think that we must need to allow retrieving the destination adapter when it's necessary to test directly on it.

To be discussed 😄.

[sstok]

Basically what we need is a mocked storage (not kept in the TestAdapter) which keeps track of the issues.

[shulard]

Nice idea, it'll help check all the things easily.