test: Error when saving outcomes related to multiple tasks

[cbaker6]

Fixes a bug when attempting to save multiple outcomes that are related to different taskUUID's. In the case of this type of save, an error is thrown since there's a verification of the taskUUIDs already being in the store.

This seems to be fixed in #727, but the test case in this PR is slightly more specific on testing the issue, let me know if you want me to close the PR.

fetchObjects is called here:

CareKit/CareKitStore/CareKitStore/CoreData/OCKStore+Outcomes.swift

Lines 150 to 151 in b16d15c

	let taskUUIDs = outcomes.map { $0.taskUUID }
	let tasks: [OCKCDTask] = try context.fetchObjects(withUUIDs: taskUUIDs)

and the error happens here:

CareKit/CareKitStore/CareKitStore/CoreData/OCKStore+Outcomes.swift

Lines 164 to 170 in b16d15c

	let taskUUID = outcome.taskUUID
	let task = tasksGroupedByUUID[taskUUID]?.first
	guard let task else {
	let msg = "Failed to find task with UUID \(taskUUID)"
	throw OCKStoreError.invalidValue(reason: msg)
	}

The grouping of taskID's handles the issue of multiple tasks being fetched.

This method looks like it's only being used by OCKOutcome, all others only require a fetch of one object, so they use fetchObject instead of fetchObjects.

The workaround is the developer will need to group OCKOutcome's by taskUUID and then manually save each group instead of trying to save all at once.

You can verify by running the unit test I added against the code before the commit.