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/integration/test_tls.py

@@ -92,10 +92,12 @@ def setup_test_data():
     generate_test_files(SOURCE_DIR, full=False)
     clean_dir(DEST_DIR)
     yield
-    shutil.rmtree(TEST_DATA_DIR, ignore_errors=True)
+    shutil.rmtree(SOURCE_DIR, ignore_errors=True)
+    shutil.rmtree(DEST_DIR, ignore_errors=True)
 
 
 class TestTLSBasic:
+    @pytest.mark.ci
     def test_tls_server_client(self, certs):
         """Basic TLS: server with cert/key, client with cert/key + CA."""
         clean_dir(DEST_DIR)