Add a better error message to the DhTest.

A typical error message with the new JDK looks like this:

java.lang.AssertionError: X expected to have bit length 1504, but has value 92958950218418996462987643843877938499735925222249647906188789799246291415800, which has bit length 256

NOKEYCHECK=True
PiperOrigin-RevId: 548591594

java/com/google/security/wycheproof/testcases/DhTest.java

@@ -277,7 +277,14 @@ private void testKeyPair(KeyPair keyPair, int expectedKeySize) {
     // should be chosen to prevent attacks.
     int minPrivateKeyBits = keySize / 2;
     BigInteger x = priv.getX();
-    assertTrue(x.bitLength() >= minPrivateKeyBits - 32);
+    assertTrue(
+        "X expected to have bit length at least "
+            + (minPrivateKeyBits - 32)
+            + ", but has value "
+            + x
+            + ", which has bit length "
+            + x.bitLength(),
+        x.bitLength() >= minPrivateKeyBits - 32);
     // TODO(bleichen): add tests for weak random number generators.
 
     // Verify the DH parameters.