Add SyncSortedList serialization test and remove SyncRoot (dotnet/corefx#34339)

Add SyncSortedList serialization test and remove SyncRoot (dotnet/corefx#34339)

* Add SyncSortedList serialization tests and remove SyncRoot

* Disable blob comparison for SyncSortedList

On UWP we get different blobs for SyncSortedList as reflection behaves
differently as the amount of types that we tests is less than in
.NET Core and .NET Framework. The order of fields that are serialized
into the payload are different in such a case.
Therefore we just disable the blob comparison for this type entirely.

Commit migrated from https://github.com/dotnet/corefx/commit/de591233c04efa019ea2aed0a8fc5181f6d4210b