differingProperties always propagated on adapter resource

[Gijsreyn]

Prerequisites

• Write a descriptive title.
• Make sure you are able to repro it on the latest version
• Search the existing issues.

Summary

When writing the docs for #848, I noticed that the differingProperties are always being propagated. For example, executing the following:

$instance = @{
    Name        = 'Spooler'
    StartupType = 'Automatic'
} | ConvertTo-Json

dsc resource test --resource PSDesiredStateConfiguration/Service --input $instance

Returns the desiredState equals true. I would expect the differingProperties to be empty. Instead, both Name and StartupType are returned.

Steps to reproduce

See description

Expected behavior

Return `differingProperties: []`

Actual behavior

Returns both properties

Error details

Environment data

Name                           Value
----                           -----
PSVersion                      5.1.19041.5848
PSEdition                      Desktop
PSCompatibleVersions           {1.0, 2.0, 3.0, 4.0...}
BuildVersion                   10.0.19041.5848
CLRVersion                     4.0.30319.42000
WSManStackVersion              3.0
PSRemotingProtocolVersion      2.3
SerializationVersion           1.1.0.1

Version

dsc 3.1.0-preview.6

Visuals

No response

[michaeltlombardi]

I think a major component of this is that the adapter doesn't fully munge the PSDSC resource output to match DSC expectations.

I think the implementation should take this resource output:

{ "inDesiredState": true }

And return it from the adapter as

{"_inDesiredState":true,"Name":"spooler","StartupType":"Automatic"}

This requires the adapter to synthetically invoke the get operation for the resource, but that should make everything more functional for users.

I believe this is also related to #267, which proposes a model for simplifying the implementation of adapters so that they are able to return data to DSC with minimal munging. The PSDSC adapters would still need to invoke both the Test() and Get() operations for classic PSDSC resources to return the appropriate JSON, but this could simplify the overall implementation and enable simpler implementations for new adapters, too.