* gnu/tests/ssh.scm (run-ssh-test): Introduce "SFTP file writing and reading".
Make 'sftp?' a keyword parameter.
(%test-openssh): Pass #:sftp? #t to 'run-ssh-test'.
* gnu/tests/ssh.scm (run-ssh-test): Introduce make-session-for-test,
call-with-connected-session and call-with-connected-session/auth.
(run-ssh-test)["connect"]: Rename to "shell command". Abstract its session
connection and authentication work into the above three functions.
