Make test classes available

[Marenz]

• Move conftest outside of dispatch namespace
• Move Dispatch.id attribute to the top
• Update dependency to use to/from datetime functions
• ~Also export the basic type Dispatch~
• Move MockServices, MockClient and Sampler to test package

[Marenz]

I'm a bit concerned about trying to sort of simulate the server instead of mocking

I generally agree with that concern, but as long as the implementation is bellow 300 lines, I felt it was a non-issue.

[Marenz]

• Named client/service Fake*
• Made service available through client .service
• Used that to add new tests that directly set or get the expected dispatches