### What changes were proposed in this pull request?
This PR aims to promote `LevelDBSuite.IntKeyType` class to a normal class to isolate `InMemoryIteratorSuite` from `LevelDBSuite`.
### Why are the changes needed?
We have the following test suite hierarchy.
```
DBIteratorSuite
- InMemoryIteratorSuite
- LevelDBIteratorSuite
```
`DBIteratorSuite.testRefWithIntNaturalKey` depends on `LevelDBSuite` and `InMemoryIteratorSuite` derived it. `InMemoryIteratorSuite` should not depend not `LevelDB`-specific stuff. This PR will make it sure.
```
public void testRefWithIntNaturalKey() throws Exception {
LevelDBSuite.IntKeyType i = new LevelDBSuite.IntKeyType();
...
```
### Does this PR introduce _any_ user-facing change?
No.
### How was this patch tested?
Pass the CIs.
```
$ build/sbt "kvstore/test"
```
Closes#32971 from dongjoon-hyun/SPARK-35824.
Authored-by: Dongjoon Hyun <dhyun@apple.com>
Signed-off-by: Dongjoon Hyun <dongjoon@apache.org>
a39f1eadb7