[system_tests] Fix flaky IAST standalone tests via library_interface_timeout

[vlad-scherbich]

Motivation

Flaky tests in test_asm_standalone.py::Test_IastStandalone_UpstreamPropagation_V2 (e.g. test_no_appsec_upstream__no_asm_event__is_kept_with_priority_1__from_0). See PROF-13796.

Root cause: traces sometimes weren't flushed to the agent before the scenario shut down the weblog. The tracer sends traces asynchronously; without enough delay, traces could still be buffered when containers stop.

Changes

• iast_standalone scenario (utils/_context/_scenarios/__init__.py): Set library_interface_timeout=15 so the tracer has 15 seconds to flush traces before the weblog shuts down.
• test_asm_standalone.py: Remove the per-setup _iast_standalone_wait_for_trace poll loop. Per-review feedback: avoid wait_for in setup functions (makes the scenario slow when many tests run). Use the global library_interface_timeout instead—one delay before shutdown achieves the same effect.

Reviewer checklist

• Anything but tests/ or manifests/ is modified ? I have the approval from R&P team
• A docker base image is modified?
  • the relevant build-XXX-image label is present
• A scenario is added, removed or renamed?
  • Get a review from R&P team

[github-actions]

CODEOWNERS have been resolved as:

utils/_context/_scenarios/__init__.py                                   @DataDog/system-tests-core

[datadog-datadog-prod-us1-2]

✅ Tests

🎉 All green!

❄️ No new flaky tests detected
🧪 All tests passed

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