torture: Expand last_ts variable in kvm-test-1-run.sh

The kvm-test-1-run.sh script says 'test -z "last_ts"' which always
evaluates to true (AKA zero) regardless of the value of the last_ts shell
variable.  This commit therefore inserts the needed dollar sign ("$").

Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>