[gardening] Mark boxmint_test slow.
[gardening] Mark boxmint_test slow.

Fixes https://github.com/dart-lang/sdk/issues/46430

TEST=boxmint_test

Change-Id: I015b9834b2609d2a73328382a9267b1416fbf51a
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/204563
Reviewed-by: Ryan Macnak <d738b7f001baf9789095d7dc549cff6de60b011d@google.com>
Commit-Queue: Alexander Aprelev <dfb74c35a357c2059971a4bf7fe26b6a7c18085d@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