test: refactor test-tls-ocsp-callback

refactor all var to either const/let
change all assert.equal to assert.strictEqual
change all assert.ok(...===...) to assert.strictEqual

PR-URL: https://github.com/nodejs/node/pull/9970


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