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