[gardening] Mark ffi/function_stress_test/1 test as slow.
[gardening] Mark ffi/function_stress_test/1 test as slow.

Change-Id: Ic5130781a75250165f92d86729c2d83c436db30e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/100280
Reviewed-by: Régis Crelier <c2bf536a10d9f2530c38e4e8da517718be8c39ce@google.com>
Commit-Queue: Régis Crelier <c2bf536a10d9f2530c38e4e8da517718be8c39ce@google.com>

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