The persistent task creates a new package TmpPkg with your CellListMap as a dependency and then tries to precompile TmpPkg. If this takes significant time, with a high probability there is some task created in CellListMap’s init function that does not terminate on exit of the precompilation. For more details, I would refer you to the Aqua docs at Persistent Tasks · Aqua.jl.
There have been some significant changes on this test in the 0.8.x release cycle as there were some flaws in it on 0.8.0.
I would recommend if bounding it to at least 0.8.5 (release from last week) fixes your problem or if it is due to downgrading some other dependency.
In general, I would say that downgrading test-only dependencies like Aqua is not that useful. At least for Aqua, there are usually multiple patch releases with some fixes and/or minor enhancements per minor release cycle.