fix(render): gate the webfont pseudo-content test on paint

generated_pseudo_content_shapes_with_the_loaded_webfont reads
DomLayout::word_ifc_items, which only exists under the paint feature, from
a plain cfg(test) module. cargo check --tests on default features
therefore failed with E0609 before any test could run, so the crate could
not be tested in the configuration the no-render archives ship.

Gate the test rather than the field access. Without paint,
TextEngine::new_with_web_fonts ignores the fonts it is given, so the
loaded face is the fallback face and the advance-width half of the test
compares a font against itself. Keeping that assertion alive would have
traded a compile error for a failing test, which is worse: one stops you,
the other reads like a regression.

Fixes #628.