Use interim disposition codes for #disposition_complete specs.  #3128.

Previously, these specs used final codes; however, Cases currently
doesn't use final codes at all.
Use Mdes::DispositionCode#success? for #disposition_complete?.  #3128.
Event#disposition_complete? -> Event#completed?.  #3128.

This commit also shortens up the examples for Event#completed?, as they
all follow the same format and fall pretty well into a table-driven
style of testing.

It's not quite as nice as I'd like (Cucumber-style tables would be more
readable), but IMO it isn't _that_ bad.  Further refinements if I think
of them.
Add event completion state to Psc::SyncLoader.  #3128.

This'll be used in OperationalImporterPscSync to detemrine whether a
corresponding scheduled activity should be closed as canceled or
occurred.