security: use exponential backoff in CertificateFetcherFromNetwork

refs: #4718, #4712

Change-Id: I8def533528b82ea00096e9d87897acf31dff24a4
diff --git a/tests/unit-tests/security/v2/validator.t.cpp b/tests/unit-tests/security/v2/validator.t.cpp
index af64557..ff33984 100644
--- a/tests/unit-tests/security/v2/validator.t.cpp
+++ b/tests/unit-tests/security/v2/validator.t.cpp
@@ -72,7 +72,8 @@
   m_keyChain.sign(data, signingByIdentity(subIdentity));
 
   VALIDATE_FAILURE(data, "All interests should get NACKed");
-  BOOST_CHECK_GT(face.sentInterests.size(), 1);
+  // 1 for the first interest, 3 for the retries on nack
+  BOOST_CHECK_EQUAL(face.sentInterests.size(), 4);
 }
 
 BOOST_AUTO_TEST_CASE(MalformedCert)