[gardening] Skip OOM test standalone/string_overflow_test during isolate stress testing

This test allocates too much memory and expects OOM.
When run in multiple isolates, this may lead to the process being
killed due to OOM.

Change-Id: Ic8ff8dab50b4e6e57f57a751c544ed1f1aa24446
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/438240
Auto-Submit: Alexander Markov <[email protected]>
Reviewed-by: Alexander Aprelev <[email protected]>

Author: alexmarkov

runtime/tests/concurrency/stress_test_list.json

@@ -3595,7 +3595,6 @@
   "../../../tests/standalone/stack_trace/rethrow_error_test.dart",
   "../../../tests/standalone/stack_trace/rethrow_nonerror_test.dart",
   "../../../tests/standalone/stack_trace/stack_trace_test.dart",
-  "../../../tests/standalone/string_overflow_test.dart",
   "../../../tests/standalone/typed_data_test.dart",
   "../../../tests/standalone/typed_data_view_test.dart",
   "../../../tests/standalone/unboxed_int_converter_test.dart"