static BucketEntry createBucketEntry(long offset, int len, ByteBuffAllocator allocator) {
    BucketEntry be = new MockBucketEntry(offset, len, allocator);
    be.setDeserializerReference(DESERIALIZER);
    return be;
  }
Override the {@link BucketEntry} so that we can set an arbitrary offset.
hbase-server/src/test/java/org/apache/hadoop/hbase/io/hfile/bucket/TestByteBufferIOEngine.java