make(1): add very basic test for archive handling

The whole code in arch.c had been uncovered by tests before.

The code coverage shows that neither the archive name nor the member
contain any $, even though archive.mk looks like it.  It could be
necessary to place the variable assignments below the dependency line,
to force late evaluation.


(rillig)

@@ -1,4 +1,4 @@
-# $NetBSD: Makefile,v 1.66 2020/07/26 22:15:36 rillig Exp $
+# $NetBSD: Makefile,v 1.67 2020/07/27 18:51:56 rillig Exp $
 #
 # Unit tests for make(1)
 #
@@ -30,6 +30,7 @@
 
 # Each test is in a sub-makefile.
 # Keep the list sorted.
+TESTS+=		archive
 TESTS+=		comment
 TESTS+=		cond-late
 TESTS+=		cond-short