test: improve crypto coverage

* call Certificate function directly
* check exception when sign option is undefined

PR-URL: https://github.com/nodejs/node/pull/11280
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/parallel/test-crypto-certificate.js

@@ -37,3 +37,6 @@ assert.strictEqual(certificate.exportChallenge(spkacFail), '');
 function stripLineEndings(obj) {
   return obj.replace(/\n/g, '');
 }
+
+// direct call Certificate() should return instance
+assert(crypto.Certificate() instanceof crypto.Certificate);

test/parallel/test-crypto-sign-verify.js

@@ -70,3 +70,10 @@ const keyPem = fs.readFileSync(common.fixturesDir + '/test_key.pem', 'ascii');
   verified = verStream.verify(certPem, s3);
   assert.strictEqual(verified, true, 'sign and verify (stream)');
 }
+
+// Test throws exception when key options is null
+{
+  assert.throws(() => {
+    crypto.createSign('RSA-SHA1').update('Test123').sign(null, 'base64');
+  }, /^Error: No key provided to sign$/);
+}