Override ContextManager.sharedInstance only in TestContextManager

There are many places calling `ContextManager.overrideSharedInstance` to
set or unset the global `ContextManager` singleton for testing purpose.
This change makes `TestContextManager` the only one responsible for
that.

This change also allows `ContextManagerMock` to be inistantiated without
affecting the `ContextManager.shared` singleton, thus avoids sharing
state betweetn unit test cases.

Author: crazytonyli

WordPress/WordPressTest/ContextManagerMock.m

@@ -14,18 +14,6 @@ @implementation ContextManagerMock
 @synthesize mainContext = _mainContext;
 @synthesize managedObjectModel = _managedObjectModel;
 
-- (instancetype)init
-{
-    self = [super init];
-    if (self) {
-        // Override the shared ContextManager
-        [ContextManager internalSharedInstance];
-        [ContextManager overrideSharedInstance:self];
-    }
-
-    return self;
-}
-
 - (NSManagedObjectModel *)managedObjectModel
 {
     return _managedObjectModel ?: [super managedObjectModel];

WordPress/WordPressTest/TestContextManager.m

@@ -10,13 +10,24 @@ - (instancetype)init
 {
     self = [super init];
     if (self) {
-        // Override the shared ContextManager
-        _stack = [[ContextManagerMock alloc] init];
+        self.stack = [[ContextManagerMock alloc] init];
     }
 
     return self;
 }
 
+- (void)setStack:(id<ManagerMock, CoreDataStack>)stack
+{
+    if (stack == _stack) {
+        return;
+    }
+
+    _stack = stack;
+    // Override the shared ContextManager
+    [ContextManager internalSharedInstance];
+    [ContextManager overrideSharedInstance:_stack];
+}
+
 - (NSManagedObjectModel *)managedObjectModel
 {
     return [_stack managedObjectModel];