svg_darkness_uses_rendered_shadow_in_non_srgb_working_space runs itself
again in a child process with --exact and checked only the exit status.
libtest exits 0 when --exact matches no test, so a rename of the test
would have left the parent passing without running the non-sRGB check.
The parent now also requires "1 passed" in the child's output, as
layout_cache_follows_fonts_added_later in crates/tools does.