Notably, the 'single-hyphen-option' and 'assign' contexts both default to the
'none' style, so before this patch, the tests would not have detected a failure
to apply those two contexts.
For other contexts, using $unused_highlight only helps detect the case where
the right highlight style (e.g., 'fg=yellow') is used as a result of applying
the wrong context — which should be a theoretical failure mode.
This is part of zsh-users/zsh-syntax-highlighting#198.
627eb71940