gh-160986: Fix flaky test_popping_python_frame_is_not_native (#161073)

* gh-160986: Fix flaky test_popping_python_frame_is_not_native

The unwinder can attach before the target process enters its main
loop, so on slow builds (e.g. UBSan) all 10,000 samples could be
taken before leaf() runs.  Wait until leaf() shows up in a sample
before counting.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

* Reuse _get_stack_trace_with_retry() to wait for leaf()

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

Author: asvetlov

Lib/test/test_external_inspection.py

@@ -2041,6 +2041,14 @@ def leaf(depth):
                 except RuntimeError:
                     continue
                 break
+            # The unwinder can attach before the main loop starts.
+            _get_stack_trace_with_retry(
+                unwinder,
+                condition=lambda trace: trace[0].threads and any(
+                    frame.funcname == "leaf"
+                    for frame in trace[0].threads[0].frame_info
+                ),
+            )
             samples = 0
             for _ in range(10_000):
                 try: