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.
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