tests: Ensure threads' hybrid stacks are complete

Ensure that the hybrid stack of an allocation performed in a Python
thread does not terminate at the first Python frame, but rather
continues and captures the native calls above it.

Signed-off-by: Matt Wozniski <mwozniski@bloomberg.net>

tests/integration/test_native_tracking.py

@@ -4,6 +4,7 @@
 import subprocess
 import sys
 import textwrap
+import threading
 from pathlib import Path
 
 import pytest
@@ -476,6 +477,34 @@ def test_hybrid_stack_in_a_thread(tmpdir, monkeypatch):
     assert expected_symbols == [stack[0] for stack in valloc.hybrid_stack_trace()][:3]
 
 
+def test_hybrid_stack_of_python_thread_starts_with_native_frames(tmp_path):
+    """Ensure there are native frames above a thread's first Python frame."""
+    # GIVEN
+    allocator = MemoryAllocator()
+    output = tmp_path / "test.bin"
+
+    def func():
+        allocator.valloc(1234)
+        allocator.free()
+
+    # WHEN
+    with Tracker(output, native_traces=True):
+        thread = threading.Thread(target=func)
+        thread.start()
+        thread.join()
+
+    # THEN
+    allocations = list(FileReader(output).get_allocation_records())
+
+    vallocs = [
+        event
+        for event in allocations
+        if event.size == 1234 and event.allocator == AllocatorType.VALLOC
+    ]
+    (valloc,) = vallocs
+    assert not valloc.hybrid_stack_trace()[-1][1].endswith(".py")
+
+
 @pytest.mark.parametrize("native_traces", [True, False])
 def test_native_tracing_header(native_traces, tmpdir):
     # GIVEN