test: replace var with const

Replace `var` with `const` in vm context for test-util-inspect.js.

PR-URL: https://github.com/nodejs/node/pull/36069


Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Daijiro Wachi <daijiro.wachi@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>