test: improve code in test-vm-symbols

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

PR-URL: https://github.com/nodejs/node/pull/10429


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