test: improve code in test-vm-preserves-property

* use const instead of var
* use assert.strictEqual instead assert.equal

PR-URL https://github.com/nodejs/node/pull/10428


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>