Add tests for having multiple tasks in flight

[omus]

making these concurrent was intentional, that way we at least stress the path where there are >1 task in flight. not the biggest deal though adn we shoudl probably be testing that separately/explicitly. – https://github.com/beacon-biosignals/ray_core_worker_julia_jll.jl/pull/47#discussion_r1307795836

[kleinschmidt]

this is done to my satisfaction with some of the tests from #126