make test: Run tests under env -i

This makes the tests more reproducable. In particular it avoids hiding
a WARN_CREATE_GLOBAL error when the dev happens to have defined that
variable in the environment (cf. next commit).

Makefile

@@ -41,7 +41,7 @@ test:
 	for test in highlighters/*; do \
 		if [ -d $$test/test-data ]; then \
 			echo "Running test $${test##*/}"; \
-			$(ZSH) -f tests/test-highlighting.zsh "$${test##*/}"; \
+			env -i QUIET=$$QUIET $(ZSH) -f tests/test-highlighting.zsh "$${test##*/}"; \
 			: $$(( result |= $$? )); \
 		fi \
 	done; \