[None][test] CBTS coverage: attribute inner unittest coverage to its entry

Nested inner pytests spawned by test_unittests_v2 run as pytest-main, so
sitecustomize skipped the CBTS_TEST_ID context switch and their in-process
coverage drained to the empty context. Switch context from CBTS_TEST_ID for
nested inner pytests too, attributing their coverage to the test-db entry
without loading cbts_plugin in the inner pytest.

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

Author: crazydemo

jenkins/scripts/cbts/coverage_utils/sitecustomize.py

@@ -92,9 +92,9 @@ def _final_save():
             name="cbts-periodic-save",
         ).start()
 
-    # In worker processes, attribute coverage to the current test via the CBTS_TEST_ID env var and a polled marker file.
+    # In worker processes and nested inner pytests, attribute coverage to the current test via the CBTS_TEST_ID env var.
     _initial_nodeid = os.environ.get("CBTS_TEST_ID", "").strip()
-    if _initial_nodeid and not _is_pytest_main:
+    if _initial_nodeid and (not _is_pytest_main or _is_nested_pytest):
         with _cov_lock:
             cov.switch_context(_initial_nodeid)