marionette: Factorize 'wait-for-file'.

* 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.