coroutines_test: Don't access a destroyed lambda
coroutines_test: Don't access a destroyed lambda

This fixes the test with clang and works around what we thing is a
defect in the standard.

Signed-off-by: Rafael Ávila de Espíndola <1975039f9046bf513b77c0c5639a0efc25f00e7f@scylladb.com>
Message-Id: <20200514161451.11803-1-1975039f9046bf513b77c0c5639a0efc25f00e7f@scylladb.com>

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