Clean up executors

[daniel-wer]

Description:

• Rename "test_pickling" to "multiprocessing_with_pickling" and "debug_sequential" to "sequential_with_pickling". No longer use a multiprocessing pool of size 1 for "sequential" executor by replacing it with the former "debug_sequential" executor.
• Clean up and speed up multiprocessing and non-scheduling executor tests.

Issues:

• followup for https://github.com/scalableminds/voxelytics/pull/3624#issuecomment-2283785067

Todos:

Make sure to delete unnecessary points or to check all before merging:

• [ ] Updated Changelog
• [ ] Updated Documentation
• [ ] Added / Updated Tests
• [ ] Considered adding this to the Examples