test(infra): run the e2e suite serially

Parallel execution on one CI runner maximized cold-start races and
resource pressure: the billing test's Envoy access-log scrape ran before
the merged proxy had served its route. Run chainsaw with --parallel 1
(as the retired bridge harness did) so the shared data plane warms
progressively and each scenario runs against a ready proxy.

Refs #279.

Author: ecv

Taskfile.test-infra.yml

@@ -422,6 +422,7 @@ tasks:
       - echo "🧪 chainsaw suite {{.TARGET}} (nso-standard={{.UPSTREAM_CLUSTER}}, nso-infra={{.DOWNSTREAM_CLUSTER}}); report {{.REPORT_PATH}}/{{.REPORT_NAME}}.json"
       - |
         {{.CHAINSAW}} test {{.TARGET}} \
+          --parallel 1 \
           --cluster nso-standard={{.TMP_DIR}}/.kind-{{.UPSTREAM_CLUSTER}}.yaml \
           --cluster nso-infra={{.TMP_DIR}}/.kind-{{.DOWNSTREAM_CLUSTER}}.yaml \
           --report-format JSON \