security: use exponential backoff in CertificateFetcherFromNetwork

refs: #4718, #4712

Change-Id: I8def533528b82ea00096e9d87897acf31dff24a4
diff --git a/tests/unit-tests/security/v2/certificate-fetcher-from-network.t.cpp b/tests/unit-tests/security/v2/certificate-fetcher-from-network.t.cpp
index e4b1ec9..5a6b395 100644
--- a/tests/unit-tests/security/v2/certificate-fetcher-from-network.t.cpp
+++ b/tests/unit-tests/security/v2/certificate-fetcher-from-network.t.cpp
@@ -119,13 +119,15 @@
 BOOST_FIXTURE_TEST_CASE_TEMPLATE(ValidateFailure, T, Failures, CertificateFetcherFromNetworkFixture<T>)
 {
   VALIDATE_FAILURE(this->data, "Should fail, as interests don't bring data");
-  BOOST_CHECK_GT(this->face.sentInterests.size(), 2);
+  // first interest + 3 retries
+  BOOST_CHECK_EQUAL(this->face.sentInterests.size(), 4);
+
   this->face.sentInterests.clear();
 
   this->advanceClocks(1_h, 2); // expire validator caches
 
   VALIDATE_FAILURE(this->interest, "Should fail, as interests don't bring data");
-  BOOST_CHECK_GT(this->face.sentInterests.size(), 2);
+  BOOST_CHECK_EQUAL(this->face.sentInterests.size(), 4);
 }
 
 BOOST_AUTO_TEST_SUITE_END() // TestCertificateFetcherFromNetwork