test: add fs-assert-encoding's test

Check the error of `assertEncoding`.
Confirmed in every place being used.(a place where `getoptions` is used)
assetEncoding: https://github.com/nodejs/node/blob/521767c88605cb6481ea98f396924e55f9dd22f4/lib/internal/fs.js#L18

PR-URL: #10913
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

test/parallel/test-fs-assert-encoding-error.js

@@ -0,0 +1,76 @@
+'use strict';
+require('../common');
+const assert = require('assert');
+const fs = require('fs');
+
+const options = 'test';
+const noop = () => {};
+const unknownEncodingMessage = /^Error: Unknown encoding: test$/;
+
+assert.throws(() => {
+  fs.readFile('path', options, noop);
+}, unknownEncodingMessage);
+
+assert.throws(() => {
+  fs.readFileSync('path', options);
+}, unknownEncodingMessage);
+
+assert.throws(() => {
+  fs.readdir('path', options, noop);
+}, unknownEncodingMessage);
+
+assert.throws(() => {
+  fs.readdirSync('path', options);
+}, unknownEncodingMessage);
+
+assert.throws(() => {
+  fs.readlink('path', options, noop);
+}, unknownEncodingMessage);
+
+assert.throws(() => {
+  fs.readlinkSync('path', options);
+}, unknownEncodingMessage);
+
+assert.throws(() => {
+  fs.writeFile('path', 'data', options, noop);
+}, unknownEncodingMessage);
+
+assert.throws(() => {
+  fs.writeFileSync('path', 'data', options);
+}, unknownEncodingMessage);
+
+assert.throws(() => {
+  fs.appendFile('path', 'data', options, noop);
+}, unknownEncodingMessage);
+
+assert.throws(() => {
+  fs.appendFileSync('path', 'data', options);
+}, unknownEncodingMessage);
+
+assert.throws(() => {
+  fs.watch('path', options, noop);
+}, unknownEncodingMessage);
+
+assert.throws(() => {
+  fs.realpath('path', options, noop);
+}, unknownEncodingMessage);
+
+assert.throws(() => {
+  fs.realpathSync('path', options);
+}, unknownEncodingMessage);
+
+assert.throws(() => {
+  fs.mkdtemp('path', options, noop);
+}, unknownEncodingMessage);
+
+assert.throws(() => {
+  fs.mkdtempSync('path', options);
+}, unknownEncodingMessage);
+
+assert.throws(() => {
+  fs.ReadStream('path', options);
+}, unknownEncodingMessage);
+
+assert.throws(() => {
+  fs.WriteStream('path', options);
+}, unknownEncodingMessage);