Mark io/async/test/certs as a content dir for ssl_session_test

Summary:
This test has been failing in travis because it is missing test/certs/tests-cert.pem
Mark it as a content dir

Reviewed By: yfeldblum

Differential Revision: D6884000

fbshipit-source-id: c7d42a4b23334781de28f1586ecbde04aea74ff2

CMakeLists.txt

@@ -564,7 +564,10 @@ if (BUILD_TESTS)
       TEST NotificationQueueTest SOURCES NotificationQueueTest.cpp
       TEST RequestContextTest SOURCES RequestContextTest.cpp
       TEST ScopedEventBaseThreadTest SOURCES ScopedEventBaseThreadTest.cpp
-      TEST ssl_session_test SOURCES SSLSessionTest.cpp
+      TEST ssl_session_test
+        CONTENT_DIR certs/
+        SOURCES
+          SSLSessionTest.cpp
       TEST writechain_test SOURCES WriteChainAsyncTransportWrapperTest.cpp
 
     DIRECTORY io/async/ssl/test/