tests: Don't discard future in test_semaphore_2
tests: Don't discard future in test_semaphore_2

Signed-off-by: Rafael Ávila de Espíndola <1975039f9046bf513b77c0c5639a0efc25f00e7f@scylladb.com>

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