test: strip pure assert_xml! from mixed .rs (dedup with HTML)

css_margin.rs (35), css_inline/inline.rs (22), css_margin_collapse.rs (4)
had assert_xml! tests that are now duplicated by HTML cases. Stripped
them, keeping only code-constructed tests (Node API). 974 tests pass
(HTML cases cover the stripped ones).

Author: TtTRz