tests: add signal_test to test list
tests: add signal_test to test list

So it runs together with everyone else.
Message-Id: <1d5c4694ef97131ea7086b99981204f1ef09b862@scylladb.com>

scylladb/seastar,dreamsxin/seastar,syuu1228/seastar,avikivity/seastar,cloudius-systems/seastar,avikivity/seastar,dreamsxin/seastar,dreamsxin/seastar,cloudius-systems/seastar,cloudius-systems/seastar,scylladb/seastar,avikivity/seastar,syuu1228/seastar,scylladb/seastar,syuu1228/seastar