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>

Author: asvetlov

Lib/test/test_external_inspection.py

@@ -2041,17 +2041,29 @@ def leaf(depth):
                 except RuntimeError:
                     continue
                 break
-            samples = 0
-            for _ in range(10_000):
+
+            def sample_names():
                 try:
                     threads = unwinder.get_stack_trace()[0].threads
                 except TRANSIENT_ERRORS:
-                    continue
+                    return None
                 if not threads:
-                    continue
-                frames = threads[0].frame_info
-                names = [frame.funcname for frame in frames]
+                    return None
+                names = [frame.funcname for frame in threads[0].frame_info]
                 if "leaf" not in names:
+                    return None
+                return names
+
+            # Wait until the target is running leaf(): the unwinder can
+            # attach before the main loop starts on slow builds.
+            for _ in busy_retry(SHORT_TIMEOUT):
+                if sample_names() is not None:
+                    break
+
+            samples = 0
+            for _ in range(10_000):
+                names = sample_names()
+                if names is None:
                     continue
                 samples += 1
                 self.assertNotIn(("leaf", "<native>"), zip(names, names[1:]))