This saves more time (as measured on my laptop):
* test_sftp: 17.7s -> 13s
* test_dirnode: 26.5s -> 20s
* test_ftp, test_configutil, test_web show negligible speedups
As before, some tests care about the number of shares, generally ones
which delete or corrupt shares and then expect to see the errors get
noticed or fixed. Those tests continue to use k=3/N=10.
