test: Modernize test-tls-peer-certificate.js

Modernize `test/parallel/test-tls-peer-certificate.js` according to the
following rules:
* `var` --> `const`/`let`
* `assert.equal` --> `assert.strictEqual`
* `assert.ok(a === b)` --> `assert.strictEqual(a, b)`

PR-URL: https://github.com/nodejs/node/pull/10014


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-by: Michael Dawson <michael_dawson@ca.ibm.com>