Use --test-depends to install test-required deps

Thanks to @ugexe for the heads up!

Author: paultcochrane

Makefile

@@ -30,5 +30,4 @@ test: install-deps
 
 install-deps:
 	@echo "*** Installing dependencies ***"
-	zef --/test --depsonly install .  # install most deps without testing them
-	zef --test --depsonly install .   # install test-required deps
+	zef --/test --test-depends --depsonly install .