Unit-ify compute_api delete tests

This makes the delete and soft_delete tests unit-like in nature,
and moves them to test_compute_api, as has been done recently
for others that we're moving to objects.

Related to blueprint compute-api-objects

Change-Id: I5e024871e5159f08cbba05fa261a9a2d08e4b8e5
Merge "Unit-ify compute_api delete tests"