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