tests: Add a 'print failures only' mode to 'make test', called 'make quiet-test'.

Fixes zsh-users/zsh-syntax-highlighting#262.

Currently, 'make quiet-test' uses Perl.  However, since it is considered a development
tool rather than a user-facing tool, users and downstream packages needn't install Perl.
Furthermore, even this dev-only dependency may be dropped in the future.

The only difference between tests/tap-filter here and the one in the issue is using
a `cat` subshell v. using 'undef $/; <STDIN>'.

Makefile

@@ -45,6 +45,9 @@ test:
 	done; \
 	exit $$result
 
+quiet-test:
+	$(MAKE) test QUIET=y
+
 perf:
 	@result=0; \
 	for test in highlighters/*; do \