[MINOR][TEST] Off -heap memory leaks for unit tests

## What changes were proposed in this pull request?
Free off -heap memory .
I have checked all the unit tests.

## How was this patch tested?
N/A

Author: liuxian <liu.xian3@zte.com.cn>

Closes #19075 from 10110346/memleak.

common/unsafe/src/test/java/org/apache/spark/unsafe/PlatformUtilSuite.java

@@ -73,5 +73,6 @@ public class PlatformUtilSuite {
     Assert.assertEquals(
       Platform.getByte(offheap.getBaseObject(), offheap.getBaseOffset()),
       MemoryAllocator.MEMORY_DEBUG_FILL_CLEAN_VALUE);
+    MemoryAllocator.UNSAFE.free(offheap);
   }
 }

core/src/test/java/org/apache/spark/memory/TaskMemoryManagerSuite.java

@@ -54,6 +54,7 @@ public class TaskMemoryManagerSuite {
     final long encodedAddress = manager.encodePageNumberAndOffset(dataPage, offset);
     Assert.assertEquals(null, manager.getPage(encodedAddress));
     Assert.assertEquals(offset, manager.getOffsetInPage(encodedAddress));
+    manager.freePage(dataPage, c);
   }
 
   @Test