Test mainContext fetching directly from PSC. Needs benchmarking esp. with merging

Sources/Setup/DataStack.swift

@@ -73,6 +73,7 @@ public final class DataStack: Equatable {
         self.migrationChain = migrationChain
         
         self.rootSavingContext.parentStack = self
+        self.mainContext.parentStack = self
         
         self.mainContext.isDataStackContext = true
     }