Spot another clang-tidy linter failure from: https://github.com/flutter/engine/pull/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.
```
â 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.*
https://github.com/flutter/flutter/issues/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
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