Add a test, add missing imports. refs #2388

Add a test, add missing imports. refs #2388

This tests ftpd, but not sftpd. Doing this sort of test on sftpd
requires the creation of a valid pubkey/privkey file pair, which is more
work than I want to do right now.

init_ftp/init_sftp were changed to interpret the configured
accounts.file as relative to the node's basedir, with
abspath_expanduser_unicode(accountfile, base=self.basedir).
This would happen naturally in a real node, since it os.chdir()s
to the basedir before doing anything. But tests don't do that.

Author: Brian Warner <warner@lothar.com>
Author: Daira Hopwood <daira@jacaranda.org>
Signed-off-by: Daira Hopwood <daira@jacaranda.org>