Deprecate TestUsersManager

Summary:
This type exists to make it easier to work with the Graph API to provide test user accounts.

A few reasons to get rid of this:

1. It's shipped as production code but really just exists for setting up integration tests. This is clearly wrong.
2. It's generally not the SDKs job to maintain specific calls to endpoints (outside of a few specific use cases such as login and app events). CoreKit should only implement the specific graph requests that are required to enable a user to form their own graph requests.
3. This is easily implemented by anyone who is currently using it. The internal CoreKit import can be replaced by
```
#import "FBSDKCoreKit_Basics.h"
#import "FBSDKCoreKit.h"
```
4. Graph API usage suggests that barely anyone is using this type. Effectively 0% usage.

Reviewed By: KylinChang

Differential Revision: D26915949

fbshipit-source-id: 9218d23f04c6008d151a142887fbcf1d0ce48ce5

Author: joesus

FBSDKCoreKit/FBSDKCoreKit/FBSDKTestUsersManager.h

@@ -28,7 +28,8 @@ NS_ASSUME_NONNULL_BEGIN
  Callback block for returning an array of FBSDKAccessToken instances (and possibly `NSNull` instances); or an error.
  */
 typedef void (^FBSDKAccessTokensBlock)(NSArray<FBSDKAccessToken *> *tokens, NSError *_Nullable error)
-NS_SWIFT_NAME(AccessTokensBlock);
+NS_SWIFT_NAME(AccessTokensBlock)
+DEPRECATED_MSG_ATTRIBUTE("AccessTokensBlock is deprecated and will be removed in the next major release");
 
 
 /**
@@ -44,6 +45,7 @@ NS_SWIFT_NAME(AccessTokensBlock);
  an app id and app secret. You will typically use this class to write unit or integration tests.
  Make sure you NEVER include your app secret in your production app.
  */
+DEPRECATED_MSG_ATTRIBUTE("FBSDKTestUsersManager is deprecated and will be removed in the next major release")
 NS_SWIFT_NAME(TestUsersManager)
 @interface FBSDKTestUsersManager : NSObject
 

FBSDKCoreKit/FBSDKCoreKit/FBSDKTestUsersManager.m

@@ -20,6 +20,10 @@
 
 #import "FBSDKCoreKit+Internal.h"
 
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wdeprecated-declarations"
+#pragma clang diagnostic ignored "-Wdeprecated-implementations"
+
 static NSString *const kFBGraphAPITestUsersPathFormat = @"%@/accounts/test-users";
 static NSString *const kAccountsDictionaryTokenKey = @"access_token";
 static NSString *const kAccountsDictionaryPermissionsKey = @"permissions";
@@ -336,3 +340,5 @@ - (void)fetchExistingTestAccountsWithAfterCursor:(NSString *)after handler:(FBSD
 }
 
 @end
+
+#pragma clang diagnostic pop