isolationtester's "dry run" mode

This patch adds a -n option to isolationtester.  With this option,
instead of running the tests, it prints the permutations that would be
run.  This is very useful for generating test specs which need to remove
permutations that are not valid (because they would run a command on a
blocked session).

Objections?

Sample run:

$ ./isolationtester -n < ~-/specs/classroom-scheduling.spec
permutation "rx1" "wy1" "c1" "ry2" "wx2" "c2"
permutation "rx1" "wy1" "ry2" "c1" "wx2" "c2"
permutation "rx1" "wy1" "ry2" "wx2" "c1" "c2"
permutation "rx1" "wy1" "ry2" "wx2" "c2" "c1"
permutation "rx1" "ry2" "wy1" "c1" "wx2" "c2"
permutation "rx1" "ry2" "wy1" "wx2" "c1" "c2"
permutation "rx1" "ry2" "wy1" "wx2" "c2" "c1"
permutation "rx1" "ry2" "wx2" "wy1" "c1" "c2"
permutation "rx1" "ry2" "wx2" "wy1" "c2" "c1"
permutation "rx1" "ry2" "wx2" "c2" "wy1" "c1"
permutation "ry2" "rx1" "wy1" "c1" "wx2" "c2"
permutation "ry2" "rx1" "wy1" "wx2" "c1" "c2"
permutation "ry2" "rx1" "wy1" "wx2" "c2" "c1"
permutation "ry2" "rx1" "wx2" "wy1" "c1" "c2"
permutation "ry2" "rx1" "wx2" "wy1" "c2" "c1"
permutation "ry2" "rx1" "wx2" "c2" "wy1" "c1"
permutation "ry2" "wx2" "rx1" "wy1" "c1" "c2"
permutation "ry2" "wx2" "rx1" "wy1" "c2" "c1"
permutation "ry2" "wx2" "rx1" "c2" "wy1" "c1"
permutation "ry2" "wx2" "c2" "rx1" "wy1" "c1"


-- 
Álvaro Herrera <alvherre@alvh.no-ip.org>