This partially addresses #7799.
Task state filesystems are contained within a subdirectory of their
parent allocation, so almost everything that existed for browsing task
state filesystems was applicable to browsing allocations, just without
the task name prepended to the path. I aimed to push this differential
handling into as few contained places as possible.
The tests also have significant overlap, so this includes an extracted
behavior to run the same tests for allocations and task states.
