* gnu/build/marionette.scm (wait-for-file): New procedure.
* gnu/tests/base.scm (run-mcron-test)[test](wait-for-file): Remove.
Pass second argument in 'wait-for-file' calls.
* gnu/tests/ssh.scm (run-ssh-test)[test](wait-for-file): Remove.
Pass second argument in 'wait-for-file' calls.
* gnu/tests/messaging.scm (run-xmpp-test)[test](guest-wait-for-file):
Remove.
Use 'wait-for-file' instead, with second argument.
* 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.