Timed pool cleanup not fully working

[paurkedal]

The caqti-miou/test-unix/test_pool_miou.ml is occasionally failing with:

[exception] File "caqti-miou/test-unix/test_pool_miou.ml", line 132, characters 2-8: Assertion failed
            Raised at Caqti_miou.Switch.run in file "caqti-miou/lib/caqti_miou.ml", line 180, characters 35-46
            Called from Dune__exe__Test_pool_miou.test.(fun).loop in file "caqti-miou/test-unix/test_pool_miou.ml", line 160, characters 6-37
            Re-raised at Miou.run in file "lib/miou.ml", line 1488, characters 23-59
            Called from Alcotest_engine__Core.Make.protect_test.(fun) in file "src/alcotest-engine/core.ml", line 186, characters 17-23
            Called from Alcotest_engine__Monad.Identity.catch in file "src/alcotest-engine/monad.ml", line 24, characters 31-35

This could be a bug in the pool implementation, since there wasn't a good test case before this.