[None][test] CBTS coverage: load cbts_plugin in inner unittest pytest

test_unittests_v2 spawns the unittest run as an inner pytest. It now adds
-p cbts_plugin when CBTS_COVERAGE_CONFIG is set, so in-process unittest
coverage is attributed per test instead of landing in the empty context.

Signed-off-by: Ivy Zhang <25222398+crazydemo@users.noreply.github.com>

Author: crazydemo

tests/integration/defs/test_unittests.py

@@ -1,4 +1,4 @@
-# SPDX-FileCopyrightText: Copyright (c) 2022-2024 NVIDIA CORPORATION & AFFILIATES. All rights reserved.
+# SPDX-FileCopyrightText: Copyright (c) 2022-2026 NVIDIA CORPORATION & AFFILIATES. All rights reserved.
 # SPDX-License-Identifier: Apache-2.0
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
@@ -147,6 +147,9 @@ def test_unittests_v2(llm_root, llm_venv, case: str, output_dir, request):
         "--periodic-batch-size=1",
         "--periodic-save-unfinished-test",
     ]
+    # Load the CBTS per-test coverage plugin in this inner pytest when CBTS is active.
+    if os.environ.get("CBTS_COVERAGE_CONFIG"):
+        command += ["-p", "cbts_plugin"]
     if test_prefix:
         command += [f"--test-prefix={test_prefix}"]