Data race related to reading/writing to AllocationTestModule.valid (#45191)

Summary:
The fix entails making `AllocationTestModule.valid` an Objective-C atomic property and funneling access to the ivar via the synthesized property getter and setter.

While the data race was present in test code, it would make it more difficult to spot more severe data races with the TSan. Also, getting rid of a data race is always good.

## Changelog:

[iOS][Fixed] - Data race related to access of `AllocationTestModule.valid`

Pull Request resolved: https://github.com/facebook/react-native/pull/45191

Test Plan: `RCTAllocationTests` will test the implementation of `AllocationTestModule`.

Reviewed By: christophpurrer

Differential Revision: D59155083

Pulled By: javache

fbshipit-source-id: e3217cffd0801377a25f04bf8ed0b4e2d1d88498

packages/rn-tester/RNTesterUnitTests/RCTAllocationTests.m

@@ -16,7 +16,7 @@
 
 @interface AllocationTestModule : NSObject <RCTBridgeModule, RCTInvalidating>
 
-@property (nonatomic, assign, getter=isValid) BOOL valid;
+@property (atomic, assign, getter=isValid) BOOL valid;
 
 @end
 
@@ -27,14 +27,14 @@ RCT_EXPORT_MODULE();
 - (instancetype)init
 {
   if ((self = [super init])) {
-    _valid = YES;
+    self.valid = YES;
   }
   return self;
 }
 
 - (void)invalidate
 {
-  _valid = NO;
+  self.valid = NO;
 }
 
 RCT_EXPORT_METHOD(test