test: improve crypto coverage

* call Certificate function directly
* check exception when sign option is undefined

PR-URL: https://github.com/nodejs/node/pull/11280


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>