test: ensure template strings don't leak through liquid

Author: jamesob

Makefile

@@ -19,3 +19,5 @@ test-slow:
 test-fast:
 	## Check for broken Markdown reference-style links that are displayed in text unchanged, e.g. [broken][broken link]
 	! find _site/ -name '*.html' | xargs grep ']\[' | grep -v skip-test | grep .
+	## Ensure that no template strings leak through liquid rendering
+	! find _site/ -name '*.html' | xargs grep '\$$(.*)'