tests: Run tests with NOUNSET

tests/test-highlighting.zsh

@@ -29,6 +29,8 @@
 # -------------------------------------------------------------------------------------------------
 
 
+setopt NOUNSET
+
 # Check an highlighter was given as argument.
 [[ -n "$1" ]] || {
   echo >&2 "Bail out! You must provide the name of a valid highlighter as argument."