Add -l to coverage testing.

tests/Makefile

@@ -8,9 +8,10 @@ PATH := $(realpath ..):$(realpath .):${PATH}
 # Defeat annoying behavior under Mac OS X - builtin echo doesn't do -n
 ECHO := /bin/echo
 
-all: regress
+check: regress
 	@echo "=== No diff output is good news."
 	@-advent -x 2>/dev/null	# Get usage message into coverage tests
+	@-advent -l /dev/null <quit.log >/dev/null
 
 .SUFFIXES: .chk