tests: Test ZSH_HIGHLIGHT_STYLES keys directly

Closes sorin-ionescu#287. Testing the keys directly removes the need for
unused_highlight which will be removed next.

tests/test-highlighting.zsh

@@ -50,6 +50,12 @@
 # Load the main script.
 . ${0:h:h}/zsh-syntax-highlighting.zsh
 
+# Overwrite _zsh_highlight_add_highlight so we get the key itself instead of the style
+_zsh_highlight_add_highlight()
+{
+  region_highlight+=("$1 $2 $3")
+}
+
 # Activate the highlighter.
 ZSH_HIGHLIGHT_HIGHLIGHTERS=($1)
 
@@ -104,8 +110,8 @@ run_test_internal() {
     [[ $highlight_zone[3] == NONE ]] && highlight_zone[3]=
     [[ -n "$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
-        echo "not ok $i ${(qqq)BUFFER[$highlight_zone[1],$highlight_zone[2]]} [$highlight_zone[1],$highlight_zone[2]]: expected ${(qqq)ZSH_HIGHLIGHT_STYLES[$highlight_zone[3]]}, observed ${(qqq)observed_result[$j]}.$todo"
+      if [[ "$observed_result[$j]" != "$highlight_zone[3]" ]]; then
+        echo "not ok $i ${(qqq)BUFFER[$highlight_zone[1],$highlight_zone[2]]} [$highlight_zone[1],$highlight_zone[2]]: expected $highlight_zone[3], observed ${(qqq)observed_result[$j]}.$todo"
         continue 2
       fi
     done

zsh-syntax-highlighting.zsh

@@ -196,9 +196,10 @@ _zsh_highlight_cursor_moved()
   [[ -n $CURSOR ]] && [[ -n ${_ZSH_HIGHLIGHT_PRIOR_CURSOR-} ]] && (($_ZSH_HIGHLIGHT_PRIOR_CURSOR != $CURSOR))
 }
 
-# Add a highlight defined by ZSH_HIGHLIGHT_STYLES
+# Add a highlight defined by ZSH_HIGHLIGHT_STYLES.
 #
-# Should be used by all highlighters aside from 'pattern' (cf. ZSH_HIGHLIGHT_PATTERN)
+# Should be used by all highlighters aside from 'pattern' (cf. ZSH_HIGHLIGHT_PATTERN).
+# Overwritten in tests/test-highlighting.zsh when testing.
 _zsh_highlight_add_highlight()
 {
   local -i start end