fix flaky test (#21078)
fix flaky test (#21078)

Signed-off-by: 654382b5c2e21ef54f49ac74d0e9fa1b331b2d46@google.com

Commit Message: TrailersWireBytesCountUpstream test is flaky because of header compression. The fix changes the workflow of the test a bit to mitigate the racing between sending trailers and sync'ing compression instruction tables.

Risk Level: n/a, test only
Testing: existing tests pass
Docs Changes: n/a
Release Notes: n/a
Platform Specific Features: n/a
envoyproxy/envoy,envoyproxy/envoy,envoyproxy/envoy,envoyproxy/envoy,envoyproxy/envoy,envoyproxy/envoy,envoyproxy/envoy,envoyproxy/envoy,envoyproxy/envoy,envoyproxy/envoy,envoyproxy/envoy,envoyproxy/envoy