Attempt to fix a deadlock in ContextManager.saveDerivedContext

When `wait` argument is true, `saveDerivedContext` method essentially
translates into this:

```
derivedContext.performBlockAndWait {
    derivedContext.save()

    mainContext.performBlockAndWait {
        mainContext.save()
    }
}
```

This change moves the inner `performBlockAndWait` out, which achieves
some thing as before - save derived context first, then save main context,
the method returns after both context are saved.

WordPress/Classes/Utility/ContextManager.m

@@ -135,8 +135,8 @@ - (void)saveDerivedContext:(NSManagedObjectContext *)context andWait:(BOOL)wait
     if (wait) {
         [context performBlockAndWait:^{
             [self internalSaveContext:context];
-            [self saveContext:self.mainContext andWait:wait withCompletionBlock:completionBlock];
         }];
+        [self saveContext:self.mainContext andWait:wait withCompletionBlock:completionBlock];
     } else {
         [context performBlock:^{
             [self internalSaveContext:context];

WordPress/WordPressTest/ContextManagerMock.m

@@ -76,15 +76,6 @@ - (NSManagedObjectContext *)mainContext
     return _mainContext;
 }
 
-- (void)saveContextAndWait:(NSManagedObjectContext *)context
-{
-    [super saveContextAndWait:context];
-    // FIXME: Remove this method to use superclass one instead
-    // This log magically resolves a deadlock in
-    // `ZDashboardCardTests.testShouldNotShowQuickStartIfDefaultSectionIsSiteMenu`
-    NSLog(@"Context save completed");
-}
-
 - (NSURL *)storeURL
 {
     NSString *documentsDirectory = [NSSearchPathForDirectoriesInDomains(NSDocumentDirectory,

WordPress/WordPressTest/ContextManagerTests.swift

@@ -115,6 +115,23 @@ class ContextManagerTests: XCTestCase {
         }
     }
 
+    func testNoDeadlockWhenSavingDerivedContext() {
+        // The purpose of this test is checking if it executes without getting
+        // stuck. That's why there is no assertion in this test method.
+        //
+        // But it the test does get stuck, the result is kind of catastrophic,
+        // as it will block the rest of the test suite from being executed.
+
+        let context = contextManager.newDerivedContext()
+        let blog = BlogBuilder(context).build()
+        let guide = QuickStartTourGuide.shared
+        guide.setup(for: blog, type: .newSite)
+        // This method calls `ContextManager.saveContextAndWait` twice with
+        // a derived context, which caused a deadlock - the second call was
+        // stuck at `performBlockAndWait`.
+        guide.remove(from: blog)
+    }
+
     // MARK: - Helper Methods
 
     fileprivate func startupCoredataStack(_ modelName: String) {