Enable cppcoreguidelines-pro-type-vararg check

We only had one usage of printf in the code that was in violation of
this rule, so replace it with iostreams, and enable the check.

Signed-off-by: Ed Tanous <edtanous@google.com>
Change-Id: Ie62165b599a996f34893aa5a3f8d1f6e6cbaf903
diff --git a/tests/test_Utils.cpp b/tests/test_Utils.cpp
index 63fcde8..a66b70c 100644
--- a/tests/test_Utils.cpp
+++ b/tests/test_Utils.cpp
@@ -65,7 +65,7 @@
              p != fs::recursive_directory_iterator(); ++p)
         {
             std::string path = p->path().string();
-            fprintf(stderr, "%s\n", path.c_str());
+            std::cerr << path << "\n";
             if (p.depth() >= 6)
             {
                 p.disable_recursion_pending();