Lib_bees: fix removed async_lwt worker initialization

Some code has been removed by mistake. This MR restore it and add a simple test in order to avoid the lwt_async worker to be disabled again.

Checkout the first commit and run

dune exec src/lib_bees/test/main.exe -- --verbose --file test_bees_lwt_async.ml

it will fail.

Checkout the second one and run the test again, it will succeed.