FetchContent_MakeAvailable (#3351)

* 🔧 use FetchContent_MakeAvailable

* 💚 fix test names

test/cmake_fetch_content2/project/main.cpp

@@ -0,0 +1,8 @@
+#include <nlohmann/json.hpp>
+
+int main(int argc, char **argv)
+{
+    nlohmann::json j;
+
+    return 0;
+}