test: update test-tls-check-server-identity.js

Changed var to const, assert.equal to assert.strictEqual, and
used a template string for error output.

PR-URL: https://github.com/nodejs/node/pull/9986


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