tests/cli(test[add]): assert --no-merge preserves duplicate sections

why: Finalize the regression to ensure vcspull add keeps duplicate workspace roots when --no-merge.
what:
- remove xfail and validate ordered items before and after the add operation

Author: tony

tests/cli/test_add.py

@@ -819,12 +819,6 @@ class NoMergePreservationFixture(t.NamedTuple):
 ]
 
 
-@pytest.mark.xfail(
-    reason=(
-        "vcspull add --no-merge overwrites earlier duplicate workspace sections "
-        "(data loss bug)"
-    ),
-)
 @pytest.mark.parametrize(
     list(NoMergePreservationFixture._fields),
     NO_MERGE_PRESERVATION_FIXTURES,