* feat: memtable backed by DataFusion to ease testing
* move test utility codes out of src folder
* Implement our own MemTable because DataFusion's MemTable does not support limit; and replace the original testing numbers table.
* fix: address PR comments
* fix: "testutil" -> "test-util"
* roll back "NumbersTable"
Co-authored-by: luofucong <luofucong@greptime.com>