Speedup MessageDigestTests#testToHexString

The purpose of this commit is to speed up the runtime of
MessageDigestTests#testToHexString. As written, the test contains a loop
that creates 1024 test cases leading to a test runtime on the order of a
few seconds. Given build infrastructure, a single test case should
suffice. Therefore, this commit removes this loop so that the test can
execute on the order of a couple hundred milliseconds.

core/src/test/java/org/elasticsearch/common/hash/MessageDigestsTests.java

@@ -59,7 +59,6 @@ public class MessageDigestsTests extends ESTestCase {
     }
 
     public void testToHexString() throws Exception {
-        for (int i = 0; i < 1024; i++) {
         BigInteger expected = BigInteger.probablePrime(256, random());
         byte[] bytes = expected.toByteArray();
         String hex = MessageDigests.toHexString(bytes);
@@ -70,5 +69,4 @@ public class MessageDigestsTests extends ESTestCase {
         BigInteger actual = new BigInteger(hex, 16);
         assertEquals(expected, actual);
     }
-    }
 }