tests: Fix issue with UnitTestClock-based event scheduling (extra sleep required)

Change-Id: I77f24f630697b6e41b3d935f0f1416e806516412
Refs: #2152
diff --git a/tests/unit-tests/security/test-certificate-cache.cpp b/tests/unit-tests/security/test-certificate-cache.cpp
index d98dd4a..54454e1 100644
--- a/tests/unit-tests/security/test-certificate-cache.cpp
+++ b/tests/unit-tests/security/test-certificate-cache.cpp
@@ -110,6 +110,7 @@
 BOOST_FIXTURE_TEST_CASE(TtlRefresh, UnitTestTimeFixture)
 {
   cache->insertCertificate(cert1); // 500ms
+
   io.poll();
   BOOST_CHECK_EQUAL(cache->getSize(), 1);
 
@@ -119,6 +120,7 @@
 
     // Refresh certificate in cache
   cache->insertCertificate(cert1); // +500ms
+
   io.poll();
   BOOST_CHECK_EQUAL(cache->getSize(), 1);
 
@@ -140,6 +142,7 @@
   BOOST_CHECK_EQUAL(cache->getSize(), 2);
 
   cache->reset();
+
   io.poll();
   BOOST_CHECK_EQUAL(cache->getSize(), 0);
 }