tests/flush_queue_test: Don't assume continuations run immediately
tests/flush_queue_test: Don't assume continuations run immediately

This patch fixes an issue with test_propagation(), where the test
assumed that after the future returned from wait_for_pending(0)
resolved, the continuations set for the post operation had already
run, which is not true.

Signed-off-by: Duarte Nunes <a60e8b7496f7715d71354fe4c06dd24017025bf1@scylladb.com>
Message-Id: <20180305131908.7667-1-a60e8b7496f7715d71354fe4c06dd24017025bf1@scylladb.com>

avikivity/scylla,duarten/scylla,scylladb/scylla,duarten/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,duarten/scylla,scylladb/scylla,avikivity/scylla