[gardening] Mark vm/dart_2/causal_stacks/async_throws_stack_lazy_non_symbolic_test as slow
[gardening] Mark vm/dart_2/causal_stacks/async_throws_stack_lazy_non_symbolic_test as slow

These tests are slow and have flaky timeouts on dartkp-*-simarm and
dartkp-*-simarm64 bots. On dartkp-linux-release-simarm64 this test
recently becomes non-flaky timeout. Trying to increase the time
allocated to run those tests.

Change-Id: I1252b19fa7dbf7e592043a76c1402a2454793b16
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/184501
Reviewed-by: Ben Konyi <67ffcaedcc3dc3d87dddb9627b72dd41756b31b1@google.com>
Commit-Queue: Alexander Markov <239e774fd29abc1267878f57afe1a8a03e0ca887@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk