Redmine #6623: Fix agents constantly retrying long running promises.

The expireafter default should be two hours, but it was erronously set to one minute in the initialization code, so remove that and stick to the globally set value.

This fixes the problem where, when transferring a large file that takes longer than the execution interval to finish, no agent will ever succeed. What happens is that if a second cf-agent starts downloading the same file, locking will not prevent the attempt, despite already being in progress by the first agent. No agent will ever succeed, because the second agent will delete the temporary file created by the first agent, then create its own temporary file, which will be deleted by the third agent, and so on.
Because of the sleep, the promise running time is two minutes, which is longer than the default ``ifelapsed`` value (1 minute). It doesn't actually influence the test itself, since it finishes in less than two minutes, but it's there to avoid processes that linger for very long, and may in fact disturb the test if you run it again shortly after. The best way would be to kill them of course, but that turned out to be tricky because of the backgrounding, so I decided two minutes maximum is not *that* bad. :-)