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: #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>

test/parallel/test-fs-read-file-assert-encoding.js

@@ -0,0 +1,13 @@
+'use strict';
+require('../common');
+
+const assert = require('assert');
+const fs = require('fs');
+
+const encoding = 'foo-8';
+const filename = 'bar.txt';
+
+assert.throws(
+  fs.readFile.bind(fs, filename, { encoding }, () => {}),
+  new RegExp(`Error: Unknown encoding: ${encoding}$`)
+);