[vm/ffi] Delete helper thread in ffi/function_gc_test
[vm/ffi] Delete helper thread in ffi/function_gc_test

Fixes memory leak on ASAN bot.

Change-Id: Idbfd1e66b703e1203fec027a040147179dd0752d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/112343
Auto-Submit: Samir Jindel <5720a19f3a84f5405fa0776737a4149175e41e6a@google.com>
Reviewed-by: Martin Kustermann <a35dd5b79fac84b2bb3b026a16b8becc55af5961@google.com>
Commit-Queue: Samir Jindel <5720a19f3a84f5405fa0776737a4149175e41e6a@google.com>

dartino/dart-sdk,dart-lang/sdk,dart-lang/sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk