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.

cmd/chasquid-util/dkim.go

@@ -82,6 +82,13 @@ func dkimVerify() {
 			})
 	}
 
+	if txt, ok := args["--txt"]; ok {
+		ctx = dkim.WithLookupTXTFunc(ctx,
+			func(ctx context.Context, domain string) ([]string, error) {
+				return []string{txt}, nil
+			})
+	}
+
 	results, err := dkim.VerifyMessage(ctx, string(msg))
 	if err != nil {
 		Fatalf("Error verifying message: %v", err)