Fix iOS unittests leak in shared.invoke method channel that causes crash (flutter/engine#45416)

Fixes https://github.com/flutter/flutter/issues/133776

"The crash is due to testShareScreenInvoked presents the activityViewController, which prevents the engineViewController to be dealloc'd in the test: https://github.com/flutter/engine/blob/main/shell/platform/darwin/ios/framework/Source/FlutterPlatformPlugin.mm#L161C4-L161C24"

This fix stubbing the presentViewController call, which prevents the viewController being alloc'd, thus preventing the leak. It is probably sufficient for this test.

[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/FlutterPlatformPluginTest.mm

@@ -139,6 +139,10 @@
   FlutterViewController* engineViewController =
       [[[FlutterViewController alloc] initWithEngine:engine nibName:nil bundle:nil] autorelease];
   FlutterViewController* mockEngineViewController = OCMPartialMock(engineViewController);
+  OCMStub([mockEngineViewController
+      presentViewController:[OCMArg isKindOfClass:[UIActivityViewController class]]
+                   animated:YES
+                 completion:nil]);
 
   FlutterPlatformPlugin* plugin =
       [[[FlutterPlatformPlugin alloc] initWithEngine:_weakFactory->GetWeakPtr()] autorelease];