web_long_running_tests is flaky

[yjbanov]

According to infra metrics it's flaky 2.08% of the time (eyeballing the LUCI dashboard though, the rate is 3/200 or 1.5%; not sure how the dashboard computes it). There's no error messages or stack traces in the logs, unfortunately. We should probably fix that first.

Sample
Sample
Sample

[fluttergithubbot]

Current flaky ratio for the past 15 days is 1.86%.
One recent flaky example for a same commit: https://ci.chromium.org/ui/p/flutter/builders/prod/Linux%20web_long_running_tests_1_5/934
Commit: b0e51db
Flaky builds:
https://ci.chromium.org/ui/p/flutter/builders/prod/Linux%20web_long_running_tests_1_5/934
https://ci.chromium.org/ui/p/flutter/builders/prod/Linux%20web_long_running_tests_1_5/908
https://ci.chromium.org/ui/p/flutter/builders/prod/Linux%20web_long_running_tests_1_5/835

[fluttergithubbot]

Current flaky ratio for the past 15 days is 1.12%.
One recent flaky example for a same commit: https://ci.chromium.org/ui/p/flutter/builders/prod/Linux%20web_long_running_tests_1_5/934
Commit: b0e51db
Flaky builds:
https://ci.chromium.org/ui/p/flutter/builders/prod/Linux%20web_long_running_tests_1_5/934
https://ci.chromium.org/ui/p/flutter/builders/prod/Linux%20web_long_running_tests_1_5/908

[fluttergithubbot]

Current flaky ratio for the past 15 days is 0.00%.

[fluttergithubbot]

Current flaky ratio for the past 15 days is 0.77%.
One recent flaky example for a same commit: https://ci.chromium.org/ui/p/flutter/builders/prod/Linux%20web_long_running_tests_1_5/1208
Commit: 91e0759
Flaky builds:
https://ci.chromium.org/ui/p/flutter/builders/prod/Linux%20web_long_running_tests_1_5/1208

[fluttergithubbot]

Current flaky ratio for the past 15 days is 2.99%.
One recent flaky example for a same commit: https://ci.chromium.org/ui/p/flutter/builders/prod/Linux%20web_long_running_tests_1_5/1254
Commit: d83fba1
Flaky builds:
https://ci.chromium.org/ui/p/flutter/builders/prod/Linux%20web_long_running_tests_1_5/1266
https://ci.chromium.org/ui/p/flutter/builders/prod/Linux%20web_long_running_tests_1_5/1254
https://ci.chromium.org/ui/p/flutter/builders/prod/Linux%20web_long_running_tests_1_5/1233
https://ci.chromium.org/ui/p/flutter/builders/prod/Linux%20web_long_running_tests_1_5/1208