tests: Use $unused_highlight in more places.

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.

highlighters/main/test-data/assign-append.zsh

@@ -28,6 +28,7 @@
 # vim: ft=zsh sw=2 ts=2 et
 # -------------------------------------------------------------------------------------------------
 
+ZSH_HIGHLIGHT_STYLES[assign]=$unused_highlight
 BUFFER='a+=(lorem ipsum)'
 
 expected_region_highlight=(