Create a hard or symbolic link. However, you would of course need to take care that testing doesn’t affect the contents of wp-content.
Another approach would be to put wp-content (or a copy) into its own file system partition (or just make a .img file from it). Then dd (or its Windows analog) should duplicate it fairly quickly.
As an aside, its worth asking if you really need a complete copy of the large set of files in wp-content. Unless you’re doing performance testing, perhaps a well-chosen subset would suffice.
added:
One thing to check is whether anything important behaves differently with a symbolic link. The one case that comes to mind is recursive deletion of the test directory tree. IIRC it will detect the existence of the symlink and delete it but not recurse into it, avoiding unintentional deletion of part of the production set-up. So here the difference is actually a good thing. (But I’d double-check the correctness of this.) (This might also be a reason to do a soft (symbolic) link instead of a hard link.)