refactor allocation test library

Refactor allocation test library to make it easier to create multiple
volumes.
use next volume which is not an archive volume
rename get_or_create to shadow_get_or_create

get_or_create is going to have to change to filter out certain
parameters, etc. which doesn't really make it a get_or_create anymore.
refactor to allow multiple params
account for kilobytes_requested in shadow_get_or_create

If an allocation already exists it can have a different
kilobytes_requested so we filter that from the get params and apply it
to the gotten allocation.