Fix `test_general` hang in case `multiprocessing` backend is used

[prutskov]

Tests in test_general.py hang in case of using a small number of workers. https://github.com/modin-project/unidist/blob/bd45958838a62576b962ca9e19b39f7fb0e41161/unidist/test/test_general.py#L15-L24

Possible reason is in incorrect finishing worker-process in multiprocessing backend.

To reproduce need to set CpuCount = 2.