futures_test: Add a couple of do_with tests
futures_test: Add a couple of do_with tests

These are already passing. They are just interesting cases I noticed
while refactoring.

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

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