Use PHPUnit for unit testing

Follow up to #413. This is not quite complete, but I'm over 90% there. 

I've splitted the pull request into two commits for now so it's easier to work with. Everything in the folder test is not converted yet. I've detailed some problems I need help with below. For the final result I intent on one big commit removing the old tests and adding the new ones. Since I'm preserving the folder structure this should give us a nice clean changelog.

While I do the remaining unit tests for Transport I need some help with the following things:

- [x] There's one acceptance test left I can't get to work. I added a @todo comment to describe the problem.
- [x] There's one unit test in the folder MIME I can't get to work with Mockery. I added a @todo comment to describe the problem.
- [ ] The smoke tests have lost the visual comparison. I have no idea how to get that to work with PHPUnit.
- [ ] The tests for the new DKIM feature still have to be done as I don't have the necessary extension installed.

If someone could look at those three things I'd appreciate it.