[system_tests] Fix flaky test_asm_standalone.py tests

[vlad-scherbich]

https://datadoghq.atlassian.net/browse/PROF-13796

Motivation

Out of the latest 100 flaked system-test CI runs on dd-trace-py main, 75 are dynamic configuration tests (58% of total). See JIRA for details.

Similar motivation as #6371. Test suite in question: test_asm_standalone.py::Test_IastStandalone_UpstreamPropagation_V2::test_no_appsec_upstream__no_asm_event__is_kept_with_priority_1__from_0

These tests are flaky because the trace sometimes wasn't flushed to the agent before the scenario stopped. The tracer sends traces asynchronously; without a short delay, the trace could still be buffered when containers shut down.

Changes

• Add a deterministic poll loop (same approach as the dynamic config fix): poll interfaces.library.get_spans(request) every 0.5s until the trace appears (5s timeout). Exits as soon as the trace is captured, or fails with a clear error on timeout.
• Add _iast_standalone_wait_for_trace(request) helper and apply it in setup for all 9 flaky IAST tests in Test_IastStandalone_UpstreamPropagation_V2. Scoped to iast_standalone only so appsec_standalone (e.g. PHP) is unchanged.

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:

tests/appsec/test_asm_standalone.py                                     @DataDog/asm-libraries @DataDog/system-tests-core

[datadog-datadog-prod-us1-2]

✅ Tests

🎉 All green!

❄️ No new flaky tests detected
🧪 All tests passed

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