Commit Message:

In SendGoAwayNotTriggerredByDecodingFilter it could happen that FakeStream will get destroyed (when the test ends) before FakeUpstream dispatcher thread (FakeUpstream::threadRoutine()) exits or before all processing on the connection is complete.

That leads to a situation when FakeUpstream dispatcher thread may try to call into the FakeStream::onResetStream when connection established by the test gets terminated after FakeStream was already destroyed.

This triggers MSAN issue on clang-18 pretty consistently, though I don't think it's deterministic and could potentially happen on clang-14 currently used by Envoy CIs.

This change makes sure that we do cleanup before exiting the test, thus preventing the use-after-free issue.

Additional Description:

Related to #37911 and fixes one of the issues that block clang-18 adoption in Envoy CI.

Risk Level: low
Testing: do_ci.sh msan using Envoy CI image with clang-18
Docs Changes: n/a
Release Notes: n/a
Platform Specific Features: n/a

Request variables

Key	Value
ref	37f93dc
sha	0c2ce96
pr	38509
base-sha	f1e276f
actor	@krinkinmu
message	Cleanup before freeing FakeStream to prevent use-after-free...
started	1740078835.559974
target-branch	main
trusted	false

Container image/s (as used in this CI run)

Envoy version (as used in this CI run)