Summary:
Add objc test for ```NSArray``` and ```NSMutableArray```.
```NSMutableArray``` is a subclass of ```NSArray```.
For documentation of ```NSArray```, https://developer.apple.com/documentation/foundation/nsarray?language=objc
For documentation of ```NSMutableArray```, https://developer.apple.com/documentation/foundation/nsmutablearray?language=objc
The underlying mechanism for ```NSMutableArray``` is quite complicated. It changes the underlying data structure during runtime, so it is possible to have say O(log n) complexity for accessing element in array. (See here https://opensource.apple.com/source/CF/CF-855.11/CFArray.h) However, this is unlikely to happen if the engineer does not abuse the usage of the class ```NSMutableArray``` according to at least two ios engineers. So here the complexity is set to match the normal expectation of the complexity.
Reviewed By: ezgicicek
Differential Revision: D22041277
fbshipit-source-id: c27f43167
Qianyi Shu