Fixes #12098 - Dangling Fog.mock! makes compute_resource_vms test fail

test/unit/compute_resource_test.rb contains a Fog.mock! without a
Fog.unmock!. This causes
test/functional/compute_resource_vms_controller_test.rb to fail only
when ran through rake test on Rails 4. Fix is as simple as adding the
missing unmock!.