[gardening] Remove lib/async/stream_periodic4_test from isolate stress-testing

This test makes assumptions about timing, so it is not suitable for
stress-testing.

TEST=ci

Change-Id: I5a74b17888277314187f19552193fe9d83e0f65f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/440000
Auto-Submit: Alexander Markov <[email protected]>
Reviewed-by: Alexander Aprelev <[email protected]>

Author: alexmarkov

runtime/tests/concurrency/stress_test_list.json

@@ -3226,7 +3226,6 @@
   "../../../tests/lib/async/stream_multi_test.dart",
   "../../../tests/lib/async/stream_periodic2_test.dart",
   "../../../tests/lib/async/stream_periodic3_test.dart",
-  "../../../tests/lib/async/stream_periodic4_test.dart",
   "../../../tests/lib/async/stream_periodic5_test.dart",
   "../../../tests/lib/async/stream_periodic6_test.dart",
   "../../../tests/lib/async/stream_periodic_test.dart",