Add Service unittests (Uilenballen per 15 tests)

[skamoen]

Payments is currently virtually untested. As we're dealing with money business, tests are important!

This issue focuses on testing the various Services. Use JUnit and AssertJ in your tests. Apply everything you've learned in the Software Quality & Testing course!

Each method in the service needs to be tested thoroughly, covering each if-statement and possible abusive input. Each test should test one single aspect. Use mocks where applicable, as each test should only focus on one aspect of one service. Don't depend on 3rd parties (such as Mollie) for your tests, tests should work offline.

You'll get 100 uilenballen for each 15 new and proper tests!