test: coroutines_test: assign spinner lambda to local variable
test: coroutines_test: assign spinner lambda to local variable

Otherwise the temporary lambda instance gets destroyed
right after the call and leads to undefined behavior

See https://bugs.llvm.org/show_bug.cgi?id=52136
for an analysis of the problem, and why it is not
a compiler bug.

Test: coroutines_test(dev)

Signed-off-by: Benny Halevy <23c997c90eb8537635e9392d48e7af1bf1b2ca22@scylladb.com>
Message-Id: <20211012112251.247246-1-23c997c90eb8537635e9392d48e7af1bf1b2ca22@scylladb.com>

syuu1228/seastar,syuu1228/seastar,scylladb/seastar,avikivity/seastar,avikivity/seastar,scylladb/seastar,syuu1228/seastar,avikivity/seastar,scylladb/seastar