ci: address review — use --dist=load, per-module teardown, exclude setpriv

- Replace --dist=loadgroup (a no-op: it only groups by xdist_group marks) with
  --dist=load, and make module teardowns remove only their own SOURCE_DIR/DEST_DIR
  (never the shared worker-keyed TEST_DATA_DIR) so interleaved modules can't wipe
  each other's fixtures. Add a session-scoped cleanup of the per-worker dir.
  (loadfile grouped the whole test_features.py module onto one worker and slowed
  the full suite to 761s vs 224s with load.)
- Mark the two setpriv privilege tests with a 'setpriv' marker and run the full
  merge suite as -m "not setpriv", so the dev/main gate can't go red on the
  env-dependent /root-traversal tests regardless of the runner uid.
- Add a TLS basic test to the ci subset, add workflow_dispatch for on-demand full
  runs, and fix trailing newlines.
- Measured: smoke -m ci = 28 passed/39s; full -n4 = 280 passed/11 skipped/1 xpassed
  in 224s (was 860s serial).

tests/conftest.py

@@ -1,17 +1,31 @@
 """Shared pytest configuration for integration tests."""
 import os
+import shutil
 import sys
 import pytest
 
 sys.path.insert(0, os.path.join(os.path.dirname(__file__), "integration"))
 
-from common import ServerManager
+from common import ServerManager, TEST_DATA_DIR
 
 
 @pytest.fixture(scope="session")
 def shared_server():
-    """One server for the entire test session. Avoids 27+ server start/stop cycles."""
+    """One server for the entire test session. Avoids 27+ server start/stop cycles.
+
+    Under pytest-xdist this session fixture is instantiated once per worker
+    process, so each worker gets its own server on an ephemeral port."""
     server = ServerManager()
     server.start()
     yield server
     server.stop()
+
+
+@pytest.fixture(scope="session", autouse=True)
+def _cleanup_worker_test_data():
+    """Remove this (worker-keyed) TEST_DATA_DIR at the end of the session.
+
+    Modules clean only their own rows; this final pass ensures the per-worker
+    directory never lingers in the working tree."""
+    yield
+    shutil.rmtree(TEST_DATA_DIR, ignore_errors=True)