tests: suppress read(-1) -> '' calls in fileobjectobserver
This stabilizes the test output of the low-level wireproto tests
between Python 2 and 3. I don't feel great about this change, but
otherwise we get a ton of extra read(-1) output on Python 3, and this
feels like a more sustainable solution.
Bonus: test-ssh-proto-unbundle.t now passes on Python 3.
Differential Revision: https://phab.mercurial-scm.org/D3733
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