test: improve assert message in test-dh-regr

PR-URL: https://github.com/nodejs/node/pull/15912
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/pummel/test-dh-regr.js

@@ -36,9 +36,14 @@ for (let i = 0; i < 2000; i++) {
   a.generateKeys();
   b.generateKeys();
 
+  const aSecret = a.computeSecret(b.getPublicKey());
+  const bSecret = b.computeSecret(a.getPublicKey());
+
   assert.deepStrictEqual(
-    a.computeSecret(b.getPublicKey()),
+    aSecret,
-    b.computeSecret(a.getPublicKey()),
+    bSecret,
-    'secrets should be equal!'
+    'Secrets should be equal.\n' +
+    `aSecret: ${aSecret.toString('base64')}\n` +
+    `bSecret: ${bSecret.toString('base64')}`
   );
 }