Split the tests

[oliverklee]

Following our deprecation policy (#453), we will need two PHPUnit configuration files first:

• one that does not allow calls to deprecated methods
• one that allows calls to deprecated methods

Also, we'll want to have two kinds of tests

• unit tests that focus on testing only one class at a time
• functional tests that test the complete system

To combine both, I propose we have three folders/namespaces for the tests:

• Tests/Functional/
• Tests/Unit/
• Tests/UnitDeprecated/

On CI, we can run all three tests on the same job (but with 3 dedicated calls) to avoid running more jobs.

For the code coverage generation, I propose we run all tests at once using the configuration that allows deprecated calls. This will save us the work of setting up the merging of multiple coverage reports.