tests/make: convert test for ':_' modifier to parse time

usr.bin/make/unit-tests/varmod-remember.exp

@@ -1,3 +1 @@
-1 2 3 1 2 3 1 2 3
-1 2 3, SAVED=3
 exit status 0

usr.bin/make/unit-tests/varmod-remember.mk

@@ -1,12 +1,20 @@
-# $NetBSD: varmod-remember.mk,v 1.3 2020/08/23 15:18:43 rillig Exp $
+# $NetBSD: varmod-remember.mk,v 1.4 2021/03/14 17:07:11 rillig Exp $
 #
 # Tests for the :_ modifier, which saves the current variable value
 # in the _ variable or another, to be used later again.
 
+.if ${1 2 3:L:_:@var@${_}@} != "1 2 3 1 2 3 1 2 3"
+.  error
+.endif
+
 # In the parameterized form, having the variable name on the right side of
 # the = assignment operator is confusing.  In almost all other situations
 # the variable name is on the left-hand side of the = operator.  Luckily
 # this modifier is only rarely needed.
+.if ${1 2 3:L:@var@${var:_=SAVED:}@} != "1 2 3"
+.  error
+.elif ${SAVED} != "3"
+.  error
+.endif
+
 all:
-	@echo ${1 2 3:L:_:@var@${_}@}
-	@echo ${1 2 3:L:@var@${var:_=SAVED:}@}, SAVED=${SAVED}