chmod -f returns nonzero exit status if the file doesn't exist, contrary to my assumption. Fix tests.

Signed-off-by: Daira Hopwood <daira@leastauthority.com>
Add support for saving storageserver/secrets/node.privkey in versions of Tahoe-LAFS that use it.

Signed-off-by: Daira Hopwood <daira@leastauthority.com>