test: expand test coverage of fs.js

* test calling truncateSync() passing a file descriptor
* test calling truncate() passing undefined as the 2nd argument

Refs: https://coverage.nodejs.org/coverage-8ab561b2432bdae3/root/fs.js.html
      (line 673 and 692)

PR-URL: https://github.com/nodejs/node/pull/10972


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