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>

tools/testing/selftests/rcutorture/bin/kvm-test-1-run.sh

@@ -227,7 +227,7 @@ then
 			must_continue=yes
 		fi
 		last_ts="`tail $resdir/console.log | grep '^\[ *[0-9]\+\.[0-9]\+]' | tail -1 | sed -e 's/^\[ *//' -e 's/\..*$//'`"
-		if test -z "last_ts"
+		if test -z "$last_ts"
 		then
 			last_ts=0
 		fi