test/t-21-dkim: Add cross-tool check against driusan/dkimverify

This patch adds a cross-tool integration check that uses
driusan/dkim's dkimverify to confirm it can verify our own DKIM signatures.

It is optional, since the tool may not be present.

test/t-21-dkim/from_B_to_A.expected

@@ -2,12 +2,18 @@ From user-a@srv-a
 Authentication-Results: srv-a
 	;spf=none (no DNS record found)
 	;dkim=pass  header.b=*
-DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed;
+DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed;
 	d=srv-b; s=sel77; *
 	h=from:subject:to:from:subject:date:to:cc:message-id;
 	bh=*
 	b=*
 	  *
+	  *
+	  *
+	  *
+	  *
+	  *
+	  *
 From: user-b@srv-b
 To: user-a@srv-a
 Subject: Feliz primavera!