pf tests: Test killing states with multiple labels

Now that we support having multiple labels on a rule ensure that we can
use each rule label to kill states.

MFC after: 2 weeks
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D29938

(cherry picked from commit 5632f585355ad380cb9ef932c419e2efdb1fc46b)