[SPARK-25290][CORE][TEST] Reduce the size of acquired arrays to avoid OOM error

## What changes were proposed in this pull request?

`BytesToBytesMapOnHeapSuite`.`randomizedStressTest` caused `OutOfMemoryError` on several test runs. Seems better to reduce memory usage in this test.

## How was this patch tested?

Unit tests.

Closes apache#22297 from viirya/SPARK-25290.

Authored-by: Liang-Chi Hsieh <[email protected]>
Signed-off-by: Wenchen Fan <[email protected]>

Author: viirya

core/src/test/java/org/apache/spark/unsafe/map/AbstractBytesToBytesMapSuite.java

@@ -379,7 +379,7 @@ public void iteratingOverDataPagesWithWastedSpace() throws Exception {
 
   @Test
   public void randomizedStressTest() {
-    final int size = 65536;
+    final int size = 32768;
     // Java arrays' hashCodes() aren't based on the arrays' contents, so we need to wrap arrays
     // into ByteBuffers in order to use them as keys here.
     final Map<ByteBuffer, byte[]> expected = new HashMap<>();
@@ -388,7 +388,7 @@ public void randomizedStressTest() {
       // Fill the map to 90% full so that we can trigger probing
       for (int i = 0; i < size * 0.9; i++) {
         final byte[] key = getRandomByteArray(rand.nextInt(256) + 1);
-        final byte[] value = getRandomByteArray(rand.nextInt(512) + 1);
+        final byte[] value = getRandomByteArray(rand.nextInt(256) + 1);
         if (!expected.containsKey(ByteBuffer.wrap(key))) {
           expected.put(ByteBuffer.wrap(key), value);
           final BytesToBytesMap.Location loc = map.lookup(