serverutil/perm: restore sequential error test functionality
serverutil/perm: restore sequential error test functionality

This was dropped by a previous refactoring.

Change-Id: I7bb74c0cd2f28ab790111d6faf3f3be4d4349786
Reviewed-on: https://upspin-review.googlesource.com/8485
Reviewed-by: Eduardo Pinheiro <c9d6fd76b920b269b838bbb70f683c6db503d8d1@gmail.com>

upspin/upspin,upspin/upspin