[tests] Test with OnlineFileSource

When we introduce OfflineFileSource, the behavior of existing tests should
not change.

test/storage/http_load.cpp

@@ -1,6 +1,6 @@
 #include "storage.hpp"
 
-#include <mbgl/storage/default_file_source.hpp>
+#include <mbgl/storage/online_file_source.hpp>
 #include <mbgl/util/chrono.hpp>
 #include <mbgl/util/run_loop.hpp>
 
@@ -10,7 +10,7 @@ TEST_F(Storage, HTTPLoad) {
     using namespace mbgl;
 
     util::RunLoop loop;
-    DefaultFileSource fs(nullptr);
+    OnlineFileSource fs(nullptr);
 
     const int concurrency = 50;
     const int max = 10000;