[test] Fixed NPEs in RocksDBStorageEngineTest (#823)

The testPartitioning and testRemovingPartitionTwice tests suffered from
contamination from previous tests in the same class.

clients/da-vinci-client/src/test/java/com/linkedin/davinci/store/rocksdb/RocksDBStorageEngineTest.java

@@ -22,6 +22,7 @@
 import java.util.Set;
 import org.testng.Assert;
 import org.testng.annotations.AfterClass;
+import org.testng.annotations.AfterMethod;
 import org.testng.annotations.BeforeClass;
 import org.testng.annotations.Test;
 
@@ -34,6 +35,7 @@ public class RocksDBStorageEngineTest extends AbstractStorageEngineTest {
   private final ReadOnlyStoreRepository mockReadOnlyStoreRepository = mock(ReadOnlyStoreRepository.class);
   private static final int versionNumber = 0;
   private static final String topicName = Version.composeKafkaTopic(storeName, versionNumber);
+  private int testCount = 0;
 
   @Override
   public void createStorageEngineForTest() {
@@ -67,6 +69,24 @@ public void cleanUp() throws Exception {
     storageService.stop();
   }
 
+  @AfterMethod
+  public void testCounter() {
+    this.testCount++;
+  }
+
+  /**
+   * Some tests require a reset if other tests have run before them, as they are sensitive to contamination.
+   *
+   * Alternatively, we could make {@link #setUp()} have {@link org.testng.annotations.BeforeMethod} and
+   * {@link #cleanUp()} have {@link AfterMethod}, though that makes the class take longer than the current approach.
+   */
+  private void reset() throws Exception {
+    if (this.testCount > 0) {
+      cleanUp();
+      setUp();
+    }
+  }
+
   @Test
   public void testGetAndPut() {
     super.testGetAndPut();
@@ -154,11 +174,13 @@ public void testGetInvalidKeys() {
 
   @Test
   public void testPartitioning() throws Exception {
+    reset();
     super.testPartitioning();
   }
 
   @Test
   public void testAddingAPartitionTwice() throws Exception {
+    reset();
     super.testAddingAPartitionTwice();
   }