Bump up shuffle.memoryFraction to make tests pass.

We'll want to revisit this before merging, since the large minimum memory
usage means that minimum memory requirements for shuffle may be fairly
high for local tests.

Author: JoshRosen

core/src/main/java/org/apache/spark/shuffle/unsafe/UnsafeShuffleExternalSorter.java

@@ -337,7 +337,7 @@ private void allocateSpaceForRecord(int requiredSpace) throws IOException {
             final long memoryAcquiredAfterSpilling = shuffleMemoryManager.tryToAcquire(PAGE_SIZE);
             if (memoryAcquiredAfterSpilling != PAGE_SIZE) {
               shuffleMemoryManager.release(memoryAcquiredAfterSpilling);
-              throw new IOException("Can't allocate memory!");
+              throw new IOException("Unable to acquire " + PAGE_SIZE + " bytes of memory");
             }
           }
         }

core/src/test/scala/org/apache/spark/shuffle/unsafe/UnsafeShuffleSuite.scala

@@ -26,5 +26,8 @@ class UnsafeShuffleSuite extends ShuffleSuite with BeforeAndAfterAll {
 
   override def beforeAll() {
     conf.set("spark.shuffle.manager", "unsafe")
+    // UnsafeShuffleManager requires at least 128 MB of memory per task in order to be able to sort
+    // shuffle records.
+    conf.set("spark.shuffle.memoryFraction", "0.5")
   }
 }