[test] Remove cachable_idempotent_test from the isolate stress test.

This test observes side-effects of call site caching from other isolates.

Change-Id: I3c0c35a30cfa0b4d8ed6dc14d997ae81e79b3375
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/456483
Reviewed-by: Alexander Aprelev <[email protected]>
Commit-Queue: Ryan Macnak <[email protected]>

Author: rmacnak-google

runtime/tests/concurrency/stress_test_list.json

@@ -10,7 +10,6 @@
   "../vm/dart/builtin_type_names_test.dart",
   "../vm/dart/byte_array_optimized_test.dart",
   "../vm/dart/byte_array_test.dart",
-  "../vm/dart/cachable_idempotent_test.dart",
   "../vm/dart/callee_side_type_checks_test.dart",
   "../vm/dart/catch_entry_state_test.dart",
   "../vm/dart/awaiter_stacks/sync_async_start_pkg_test_test.dart",