test: refactor test-fs-read-*

* Use `common.mustNotCall()` in place of `common.noop` where appropriate
* Increase specificity of regular expressions (that is, make them match
  the whole error string rather than part of the error string) in
  `assert.throws()` calls

PR-URL: https://github.com/nodejs/node/pull/13501
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>

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

@@ -1,6 +1,6 @@
 'use strict';
-require('../common');
 
+const common = require('../common');
 const assert = require('assert');
 const fs = require('fs');
 
@@ -8,6 +8,6 @@ const encoding = 'foo-8';
 const filename = 'bar.txt';
 
 assert.throws(
-  fs.readFile.bind(fs, filename, { encoding }, () => {}),
-  new RegExp(`Error: Unknown encoding: ${encoding}$`)
+  fs.readFile.bind(fs, filename, { encoding }, common.mustNotCall()),
+  new RegExp(`^Error: Unknown encoding: ${encoding}$`)
 );