make(1): add test for suppressing the output of the command using @

usr.bin/make/unit-tests/sh-leading-at.exp

@@ -1 +1,5 @@
+ok
+space after @
+echo 'echoed'
+echoed
 exit status 0

usr.bin/make/unit-tests/sh-leading-at.mk

@@ -1,9 +1,10 @@
-# $NetBSD: sh-leading-at.mk,v 1.2 2020/08/16 14:25:16 rillig Exp $
+# $NetBSD: sh-leading-at.mk,v 1.3 2020/08/22 09:16:08 rillig Exp $
 #
 # Tests for shell commands preceded by an '@', to suppress printing
 # the command to stdout.
 
-# TODO: Implementation
-
 all:
-	@:;
+	@
+	@echo 'ok'
+	@ echo 'space after @'
+	echo 'echoed'