tests: log sendall() operations and port test-http-bad-server.t Python 3's HTTP server layer buffers output and uses sendall() instead of write(). In order to make test-http-bad-server.t pass on Python 3, we needed to teach our socket proxy to log sendall() events and to abort future sends if we reached our send limit. The tests using `tail` were difficult to port with inline output conditionals since the number of lines varied. So we now use `#if py3` for these tests. test-http-bad-server.t now passes on Python 3.6 and 3.7 on at least Linux. However, it does not yet pass on Python 3.5 because of low-level differences to how the HTTP server is implemented. Differential Revision: https://phab.mercurial-scm.org/D5753