Add test for duplicatively scheduled data migrations

[alanking]

Multiple migrations for a single tier-out or restage was addressed in #108 (see also: #103). We need to add a test to ensure that the behavior is correct in automated testing.

Once confirmed, we can remove this TODO line:

irods_capability_storage_tiering/storage_tiering.cpp

Line 804 in b50254b

// TODO:: query for already queued movement?