[SPARK-35824][CORE][TESTS] Convert LevelDBSuite.IntKeyType from a nested class to a normal class

### 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>

common/kvstore/src/test/java/org/apache/spark/util/kvstore/DBIteratorSuite.java

@@ -380,7 +380,7 @@ public abstract class DBIteratorSuite {
 
   @Test
   public void testRefWithIntNaturalKey() throws Exception {
-    LevelDBSuite.IntKeyType i = new LevelDBSuite.IntKeyType();
+    IntKeyType i = new IntKeyType();
     i.key = 1;
     i.id = "1";
     i.values = Arrays.asList("1");

common/kvstore/src/test/java/org/apache/spark/util/kvstore/IntKeyType.java

@@ -0,0 +1,47 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.spark.util.kvstore;
+
+import java.util.List;
+
+public class IntKeyType {
+
+  @KVIndex
+  public int key;
+
+  @KVIndex("id")
+  public String id;
+
+  public List<String> values;
+
+  @Override
+  public boolean equals(Object o) {
+    if (o instanceof IntKeyType) {
+      IntKeyType other = (IntKeyType) o;
+      return key == other.key && id.equals(other.id) && values.equals(other.values);
+    }
+    return false;
+  }
+
+  @Override
+  public int hashCode() {
+    return id.hashCode();
+  }
+
+}
+

common/kvstore/src/test/java/org/apache/spark/util/kvstore/LevelDBSuite.java

@@ -338,31 +338,4 @@ public class LevelDBSuite {
 
     return count;
   }
-
-  public static class IntKeyType {
-
-    @KVIndex
-    public int key;
-
-    @KVIndex("id")
-    public String id;
-
-    public List<String> values;
-
-    @Override
-    public boolean equals(Object o) {
-      if (o instanceof IntKeyType) {
-        IntKeyType other = (IntKeyType) o;
-        return key == other.key && id.equals(other.id) && values.equals(other.values);
-      }
-      return false;
-    }
-
-    @Override
-    public int hashCode() {
-      return id.hashCode();
-    }
-
-  }
-
 }