[ios] Fix testDeallocated failing locally. (flutter/engine#45663)

Moving the code block to autorelease pool to ensure the FlutterEngine is released in the test.

Fixes: https://github.com/flutter/flutter/issues/134388

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style

engine/src/flutter/shell/platform/darwin/ios/framework/Source/FlutterEngineTest.mm

@@ -113,7 +113,7 @@ FLUTTER_ASSERT_ARC
 
 - (void)testDeallocated {
   __weak FlutterEngine* weakEngine = nil;
-  {
+  @autoreleasepool {
     FlutterEngine* engine = [[FlutterEngine alloc] initWithName:@"foobar"];
     weakEngine = engine;
     [engine run];