test: improve coverage in test-crypto.dh

PR-URL: https://github.com/nodejs/node/pull/11253
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/pummel/test-crypto-dh.js

@@ -8,15 +8,32 @@ if (!common.hasCrypto) {
   return;
 }
 
-assert.throws(function() {
+assert.throws(
+  function() {
     crypto.getDiffieHellman('unknown-group');
-});
-assert.throws(function() {
+  },
+  /^Error: Unknown group$/,
+  'crypto.getDiffieHellman(\'unknown-group\') ' +
+  'failed to throw the expected error.'
+);
+assert.throws(
+  function() {
     crypto.getDiffieHellman('modp1').setPrivateKey('');
-});
-assert.throws(function() {
+  },
+  new RegExp('^TypeError: crypto\\.getDiffieHellman\\(\\.\\.\\.\\)\\.' +
+  'setPrivateKey is not a function$'),
+  'crypto.getDiffieHellman(\'modp1\').setPrivateKey(\'\') ' +
+  'failed to throw the expected error.'
+);
+assert.throws(
+  function() {
     crypto.getDiffieHellman('modp1').setPublicKey('');
-});
+  },
+  new RegExp('^TypeError: crypto\\.getDiffieHellman\\(\\.\\.\\.\\)\\.' +
+  'setPublicKey is not a function$'),
+  'crypto.getDiffieHellman(\'modp1\').setPublicKey(\'\') ' +
+  'failed to throw the expected error.'
+);
 
 var hashes = {
   modp1: '630e9acd2cc63f7e80d8507624ba60ac0757201a',