Fix running packages preloading tests in interactive mode

This change also makes the test more useful, as it verifies that reading
metadata from package.json's `packagesCache` works correctly.

src/package-manager.coffee

@@ -73,6 +73,8 @@ class PackageManager
     @loadedPackages = {}
     @preloadedPackages = {}
     @packageStates = {}
+    @packagesCache = packageJSON._atomPackages ? {}
+    @packageDependencies = packageJSON.packageDependencies ? {}
     @triggeredActivationHooks.clear()
 
   ###