test: use spread object

Object.assign() can be replaced by spread objects

PR-URL: https://github.com/nodejs/node/pull/30423
Refs: https://eslint.org/docs/rules/prefer-object-spread
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>

test/parallel/test-https-agent-additional-options.js

@@ -63,7 +63,7 @@ function variations(iter, port, cb) {
         // Save `value` for check the next time.
         value = next.value.val;
         const [key, val] = next.value;
-        https.get(Object.assign({}, getBaseOptions(port), { [key]: val }),
+        https.get({ ...getBaseOptions(port), [key]: val },
                   variations(iter, port, cb));
       }
     }));