torture: Make kvm.sh "--dryrun sched" summarize number of builds

Knowing the number of builds that kvm.sh will split a run into allows
estimation of the duration of a test, give or take build duration.
This commit therefore adds a line of output to "--dryrun sched" that
gives the number of builds that will be run.  This excludes "builds"
for repeated scenarios that reuse an earlier build.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>