test-lfs: add tests around corrupted lfs objects These are mostly tests against file:// based remote stores, because that's what we have the most control over. The test uploading a corrupt blob to lfs-test-server demonstrates an overly broad exception handler in the retry loop. A corrupt blob is actually transferred in a download, but eventually caught when it is accessed (only after it leaves the corrupt file in a couple places locally). I don't think we want to trust random 3rd party implementations, and this would be a problem if there were a `debuglfsdownload` command that simply cached the files. And given the cryptic errors, we should probably validate the file hash locally before uploading, and also after downloading.