Avoid deprecated Data::getSignature()

Change-Id: Id644d5bd4414ed97d70ed108637516199d806be9
diff --git a/tests/identity-management-fixture.cpp b/tests/identity-management-fixture.cpp
index a9251b4..db99d5a 100644
--- a/tests/identity-management-fixture.cpp
+++ b/tests/identity-management-fixture.cpp
@@ -92,7 +92,7 @@
 
   v2::AdditionalDescription description;
   description.set("type", "sub-certificate");
-  info.appendTypeSpecificTlv(description.wireEncode());
+  info.addCustomTlv(description.wireEncode());
 
   m_keyChain.sign(request, signingByIdentity(issuer).setSignatureInfo(info));
   m_keyChain.setDefaultCertificate(subIdentity.getDefaultKey(), request);