tests: store protocol payload in files
Upcoming changes to version 2 of the SSH protocol will introduce
binary components to the protocol. It will be easier to eliminate
trailing newlines and use binary in the tests if the protocol
payload is being generated by Python.
So use inline Python to write payloads to files and pipe those files
to server processes instead of shell strings/variables.
Differential Revision: https://phab.mercurial-scm.org/D2381
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