test: expand test coverage of fs.js

* test reading a file passing a not valid enconding

Refs: https://coverage.nodejs.org/coverage-067be658f966dafe/root/internal/fs.js.html
PR-URL: https://github.com/nodejs/node/pull/10947


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>