[gardening] Import NNBD version of FFI in NNBD thread_pool_test.

Change-Id: Ia44c34915939dff338f50e7b765f47a66a9bbe09
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/153961
Reviewed-by: Daco Harkes <dacoharkes@google.com>
Reviewed-by: Johnni Winther <johnniwinther@google.com>
Commit-Queue: Aske Simon Christensen <askesc@google.com>

runtime/tests/vm/dart/isolates/thread_pool_test.dart

@@ -11,7 +11,7 @@ import 'dart:ffi';
 import 'package:expect/expect.dart';
 
 import 'test_utils.dart';
-import '../../../../../tests/ffi_2/dylib_utils.dart';
+import '../../../../../tests/ffi/dylib_utils.dart';
 
 // This should be larger than max-new-space-size/tlab-size.
 const int threadCount = 200;