commitlog_test: Fix reader test dropping rp handles
commitlog_test: Fix reader test dropping rp handles

Test wants data in live segments to read from, so should
not just drop the handles returned from allocate.
Message-Id: <c8fe0a8bf7ea6e6fca977250b23c5c95dddaf91c@scylladb.com>

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