test: refactor test-util-inspect

* favor `assert.strictEqual()` and friends of `assert.equal()` etc.
* favor `.includes()` over `.indexOf()` for existence checks

PR-URL: #8189
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>
Reviewed-By: Michaël Zasso <mic.besace@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>