ScheduledTask: Add Test to cover 'StopExisting' value in MultipleInstances Parameter

[PlagueHO]

The StopExisting value in the MultipleInstances parameter must be handled in a special way because it is missing from the ENUM used by the underlying cmdlets/objects.

We don't have unit tests that specifically cover the use of this, so either extract the handling into a function that can be mocked and/or add an integration test.

See https://github.com/dsccommunity/ComputerManagementDsc/pull/335