test: refactor test-tls-ecdh-disable

* use common.mustCall()
* use String.prototype.includes() instead of String.prototype.indexOf()

PR-URL: https://github.com/nodejs/node/pull/9989


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