test: fix flaky test-crypto-classes.js

On non-FIPS, we can instantiate DiffieHellman with 256 instead of 1024.
This should be quite a bit faster, and therefore prevent the timeouts.

Backport-PR-URL: #16245
PR-URL: #15662
Fixes: #15655
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>

Author: bengl

test/parallel/test-crypto-classes.js

@@ -25,6 +25,7 @@ const TEST_CASES = {
 if (!common.hasFipsCrypto) {
   TEST_CASES.Cipher = ['aes192', 'secret'];
   TEST_CASES.Decipher = ['aes192', 'secret'];
+  TEST_CASES.DiffieHellman = [256];
 }
 
 function entries(obj) {