test harness: Don't leak options from test files to the test harness.

Fixes an issue whereby the '# TODO "issue #687"' directive in the output of
opt-shwordsplit1.zsh was truncated, because the test itself had set the
SH_WORD_SPLIT option and that affected the evaluation of
«${(z)expected_region_highlight[i]}» in the test harness.

Furthermore, this patch also independently fixes the error under
zsh-5.0.8 and earlier that was fixed by the previous commit.

tests/test-highlighting.zsh

@@ -123,7 +123,12 @@ run_test_internal() {
   local skip_mismatch
   local -a expected_region_highlight region_highlight
 
-  . "$srcdir"/"$1"
+  local ARG="$1"
+  () {
+    setopt localoptions
+    . "$srcdir"/"$ARG"
+
+    # WARNING: The remainder of this anonymous function will run with the test's options in effect
 
     (( $#skip_test )) && { print -r -- "1..0 # SKIP $skip_test"; return; }
 
@@ -137,6 +142,8 @@ run_test_internal() {
 
     # Process the data.
     _zsh_highlight
+  }
+  unset ARG
 
   if (( unsorted )); then
     region_highlight=("${(@n)region_highlight}")