no mock for custom_fields
vApp get_execute_query 'get by name' mock

This allows us to test the vapp model 'get by name' mixin in Mock mode.
Fix error in assumption about TaskRecord queries

Task records as returned by the Query API have 'name' parameter
equal to the 'operation_name' parameter of the Model object (and
OperationName value of the get_task request)

This commit documents that difference in the test, and corrects
the get_execute_query Mock to mirror the behaviour.
Replace Fog::Time with Time

For some reason, our tests are now returning regular Time
objects in both Real and Mock mode.