Flaky timeouts on devtools_server_test on all platforms

[whesse]

The test pkg/dds/test/devtools_server/devtools_server_test is timing out flakily on linux and mac (it is skipped on Windows).
It also has flaky runtime errors on these platforms.

On linux debug it has now changed to timing out stably.
So this test is never reporting any new failures, since it is already failing or flaky on all platforms it runs on.

Current results:
https://dart-ci.firebaseapp.com/current_results/#/filter=pkg/dds/test/devtools_server/devtools_server_test&showAll

Log of a runtime failure:
https://logs.chromium.org/logs/dart/buildbucket/cr-buildbucket/8818721976187542225/+/u/test_results/ignored_flaky_test_failure_logs

go/dart-flakiness can be used to inspect the flakiness of this test.

@kenzieschmoll @bkonyi