test: remove require('buffer') on 6 fs test files

* test/parallel/test-fs-mkdtemp.js
* test/parallel/test-fs-read-zero-length.js
* test/parallel/test-fs-read.js
* test/parallel/test-fs-whatwg-url.js
* test/parallel/test-fs-write-string-coerce.js
* test/parallel/test-fs-write.js

PR-URL: #13845
Refs: #13836
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Alexey Orlenko <eaglexrlnk@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>

test/parallel/test-fs-mkdtemp.js

@@ -4,7 +4,6 @@ const common = require('../common');
 const assert = require('assert');
 const fs = require('fs');
 const path = require('path');
-const Buffer = require('buffer').Buffer;
 
 common.refreshTmpDir();
 

test/parallel/test-fs-write-string-coerce.js

@@ -2,7 +2,6 @@
 const common = require('../common');
 const assert = require('assert');
 const path = require('path');
-const Buffer = require('buffer').Buffer;
 const fs = require('fs');
 
 common.refreshTmpDir();

test/parallel/test-fs-write.js

@@ -2,7 +2,6 @@
 const common = require('../common');
 const assert = require('assert');
 const path = require('path');
-const Buffer = require('buffer').Buffer;
 const fs = require('fs');
 const fn = path.join(common.tmpDir, 'write.txt');
 const fn2 = path.join(common.tmpDir, 'write2.txt');