[test] Mark vm/cc/ArrayTruncationRaces as Slow.

Change-Id: I2d0a2d4027599212f0517421181a05e9296f388b
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/437103
Reviewed-by: Siva Annamalai <[email protected]>
Commit-Queue: Ryan Macnak <[email protected]>

Author: rmacnak-google

runtime/tests/vm/vm.status

@@ -2,6 +2,7 @@
 # for details. All rights reserved. Use of this source code is governed by a
 # BSD-style license that can be found in the LICENSE file.
 
+cc/ArrayTruncationRaces: Pass, Slow
 cc/Fail0: Fail # These tests are expected to crash on all platforms.
 cc/Fail1: Fail # These tests are expected to crash on all platforms.
 cc/Fail2: Fail # These tests are expected to crash on all platforms.