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_features.py

@@ -25,7 +25,11 @@ def setup_test_data():
     generate_test_files(SOURCE_DIR, full=False)
     clean_dir(DEST_DIR)
     yield
-    shutil.rmtree(TEST_DATA_DIR, ignore_errors=True)
+    # Remove only this module's own dirs.  Under pytest-xdist the whole
+    # (worker-keyed) TEST_DATA_DIR is shared with concurrently-interleaved
+    # modules, so never rmtree it here.
+    shutil.rmtree(SOURCE_DIR, ignore_errors=True)
+    shutil.rmtree(DEST_DIR, ignore_errors=True)
 
 
 class TestDryRun:
@@ -2855,6 +2859,7 @@ class TestDeletePolicy:
                                "--clear-groups"] + cmd, text=True, capture_output=True)
 
     @pytest.mark.parametrize("mt", [False, True])
+    @pytest.mark.setpriv
     def test_ignore_errors_keeps_deletion_active_on_scan_error(self, mt):
         """A source I/O error (unreadable subdirectory) aborts the run so no
         deletion happens by default; --ignore-errors continues, still transfers
@@ -2901,6 +2906,7 @@ class TestDeletePolicy:
 
     @pytest.mark.parametrize("mt", [False, True])
     @pytest.mark.parametrize("timing", ["--delete", "--delete-before"])
+    @pytest.mark.setpriv
     def test_ignore_errors_unreadable_root_never_deletes(self, mt, timing):
         """An unreadable SOURCE ROOT must never be treated as a skippable scan
         error: with --ignore-errors the sequential scanner treats the root as