tests: fix use-after-free in sstable test
tests: fix use-after-free in sstable test

After commit a843aea547, a gate was introduced to make sure that
an asynchronous operation is finished before column family is
destroyed. A sstable testcase was not stopping column family,
instead it just removed column family from compaction manager.
That could cause an user-after-free if column family is destroyed
while the asynchronous operation is running. Let's fix it by
stopping column family in the test.

Signed-off-by: Raphael S. Carvalho <1e2942cea40e3b4d2162dfcd9e53e41c05a84d22@scylladb.com>
Message-Id: <ed910ec459c1752148099e6dc503e7f3adee54da.1461177411.git.1e2942cea40e3b4d2162dfcd9e53e41c05a84d22@scylladb.com>

avikivity/scylla,kjniemi/scylla,avikivity/scylla,duarten/scylla,kjniemi/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,raphaelsc/scylla,scylladb/scylla,duarten/scylla,raphaelsc/scylla,scylladb/scylla,raphaelsc/scylla,kjniemi/scylla,duarten/scylla