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 <[email protected]>
Reviewed-By: Michaël Zasso <[email protected]>
Reviewed-By: James M Snell <[email protected]>