tests: Run tests in an anon function to catch global variables

This would have caught #593.

tests/test-highlighting.zsh

@@ -33,6 +33,7 @@ setopt NO_UNSET WARN_CREATE_GLOBAL
 
 local -r root=${0:h:h}
 
+(){
 # Check an highlighter was given as argument.
 [[ -n "$1" ]] || {
   echo >&2 "Bail out! You must provide the name of a valid highlighter as argument."
@@ -219,3 +220,4 @@ for data_file in $root/highlighters/$1/test-data/*.zsh; do
 done
 
 exit $something_failed
+} "$@"