tests: Use 'NONE' to denote no highlighting

'none' is a valid highlighting type. Since by convention styles are all
lowercase, 'NONE' shouldn't conflict with anything.

Closes #289.

highlighters/brackets/test-data/quoted-patentheses.zsh

@@ -30,5 +30,5 @@
 BUFFER='echo "foo ( bar"'
 
 expected_region_highlight=(
-"1  16 none" # We expect the brackets highlighter to do nothing
+"1  16 NONE" # We expect the brackets highlighter to do nothing
 )

tests/test-highlighting.zsh

@@ -101,6 +101,7 @@ run_test_internal() {
   for ((i=1; i<=${#expected_region_highlight}; i++)); do
     local todo=
     highlight_zone=${(z)expected_region_highlight[$i]}
+    [[ $highlight_zone[3] == NONE ]] && highlight_zone[3]=
     [[ -n "$highlight_zone[4]" ]] && todo=" # TODO $highlight_zone[4]"
     for j in {$highlight_zone[1]..$highlight_zone[2]}; do
       if [[ "$observed_result[$j]" != "$ZSH_HIGHLIGHT_STYLES[$highlight_zone[3]]" ]]; then