tests: Fix failure of NameServer and Validator test suites

The fix makes sure that DummyFace::receive does not called from within
signal invocation.

Change-Id: Idc6d2866ece585932687c0bafe7731a6fa66ddea
diff --git a/tests/unit/validator.cpp b/tests/unit/validator.cpp
index e51782b..09af09d 100644
--- a/tests/unit/validator.cpp
+++ b/tests/unit/validator.cpp
@@ -132,7 +132,9 @@
     NDNS_LOG_TRACE("validator needs: " << certName);
     BOOST_CHECK_EQUAL(m_keyChain.doesCertificateExist(certName), true);
     auto cert = m_keyChain.getCertificate(certName);
-    m_face->receive<Data>(*cert);
+    m_face->getIoService().post([this, cert] {
+        m_face->receive<Data>(*cert);
+      });
   }
 
 public: