Make use of `init` and `cleanup` in the playlist tests

Reason for the change

Most of these tests contain the same 50--70 lines of boilerplate code to set up the test environment. Instead of copy & pasting this for each new test-case, use the special init and cleanup methods that Qt runs before and after each test.

The diff is massive because (1) I removed about 50--70 lines from each test-case, and (2) almost all variables are now pointers (so lots of changes from . to ->). I don't think I changed the functionality of any test though.

Test plan

Run cmake -R playlist