Owns the Mob.State that concurrently running Mob.ScreenCase tests share.

Mob.State is one globally named process over a globally named :dets table, so async screen tests cannot each start their own. Checking for it and starting it from every test raced: two tests saw no process, and the second start failed with :already_started. A store one test started was also stopped when that test ended, even while another test was still using it.

Every checkout and check-in goes through this process, so they are serialised. The first checkout starts Mob.State against a throwaway MOB_DATA_DIR, overlapping checkouts share it, and the last check-in stops it and restores the variable. Mob.ScreenCase checks in from on_exit, which ExUnit runs before it moves on, so tests that run afterwards (including async: false ones that start Mob.State themselves) find it gone. A Mob.State something else already started is shared but never stopped here. Started on first use and unlinked, so it outlives any single test.