fix rake test task to use leveldb.so in ext/
fix rake test task to use leveldb.so in ext/

assispedro/leveldb-ruby,assispedro/leveldb-ruby,wmorgan/leveldb-ruby,assispedro/leveldb-ruby,wmorgan/leveldb-ruby,assispedro/leveldb-ruby,wmorgan/leveldb-ruby,wmorgan/leveldb-ruby