[ios]fix unused variable clang tidy warning (#56637)

Spot another clang-tidy linter failure from: #56631

In release mode, if we remove NSAssert, then weakFlutterEngine is not used at all. This should have been an XCTAssert rather than NSAssert in the first place. 

```
� Failures for clang-tidy on /Volumes/Work/s/w/ir/cache/builder/src/flutter/shell/platform/darwin/ios/framework/Source/FlutterTextInputPluginTest.mm:
/Volumes/Work/s/w/ir/cache/builder/src/flutter/shell/platform/darwin/ios/framework/Source/FlutterTextInputPluginTest.mm:239:5: error: Value stored to 'weakFlutterEngine' is never read [clang-analyzer-deadcode.DeadStores,-warnings-as-errors]
  239 |     weakFlutterEngine = flutterEngine;
      |     ^                   ~~~~~~~~~~~~~
/Volumes/Work/s/w/ir/cache/builder/src/flutter/shell/platform/darwin/ios/framework/Source/FlutterTextInputPluginTest.mm:239:5: note: Value stored to 'weakFlutterEngine' is never read
  239 |     weakFlutterEngine = flutterEngine;
      |     ^                   ~~~~~~~~~~~~~
Suppressed 9240 warnings (9111 in non-user code, 129 NOLINT).
Use -header-filter=.* to display errors from all non-system headers. Use -system-headers to display errors from system headers as well.
```

*List which issues are fixed by this PR. You must list at least one issue.*
flutter/flutter#157837

*If you had to change anything in the [flutter/tests] repo, include a link to the migration guide as per the [breaking change policy].*

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

shell/platform/darwin/ios/framework/Source/FlutterTextInputPluginTest.mm

@@ -237,7 +237,7 @@ - (void)testNoDanglingEnginePointer {
   @autoreleasepool {
     FlutterEngine* flutterEngine = OCMClassMock([FlutterEngine class]);
     weakFlutterEngine = flutterEngine;
-    NSAssert(weakFlutterEngine, @"flutter engine must not be nil");
+    XCTAssertNotNil(weakFlutterEngine, @"flutter engine must not be nil");
     FlutterTextInputPlugin* flutterTextInputPlugin = [[FlutterTextInputPlugin alloc]
         initWithDelegate:(id<FlutterTextInputDelegate>)flutterEngine];
     weakFlutterTextInputPlugin = flutterTextInputPlugin;
@@ -254,8 +254,8 @@ - (void)testNoDanglingEnginePointer {
     currentView = flutterTextInputPlugin.activeView;
   }
 
-  NSAssert(!weakFlutterEngine, @"flutter engine must be nil");
-  NSAssert(currentView, @"current view must not be nil");
+  XCTAssertNil(weakFlutterEngine, @"flutter engine must be nil");
+  XCTAssertNotNil(currentView, @"current view must not be nil");
 
   XCTAssertNil(weakFlutterTextInputPlugin);
   // Verify that the view can no longer access the deallocated engine/text input plugin