Use mainContext instead of contextManager.mainContext

Co-authored-by: Gio Lodi <giovanni.lodi42@gmail.com>

Author: crazytonyli

WordPress/WordPressTest/ReaderStreamViewControllerTests.swift

@@ -5,7 +5,7 @@ import XCTest
 class ReaderStreamViewControllerTests: CoreDataTestCase {
     // Tests that a ReaderStreamViewController is returned
     func testControllerWithTopic() {
-        let context = contextManager.mainContext
+        let context = mainContext
         let topic = NSEntityDescription.insertNewObject(forEntityName: "ReaderTagTopic", into: context) as! ReaderTagTopic
         topic.path = "foo"