test: remove `require('buffer')` from 4 test files

We don't use the global Buffer throughout the lib/ to avoid circular
dependency issues in core, but we actually don't need to require it on
test files. So remove them on:

+ test/parallel/test-stream-uint8array.js
+ test/parallel/test-stream2-finish-pipe.js
+ test/parallel/test-tls-session-cache.js
+ test/parallel/test-vm-cached-data.js

Refs: https://github.com/nodejs/node/issues/13836
PR-URL: https://github.com/nodejs/node/pull/13844
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Alexey Orlenko <eaglexrlnk@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>

test/parallel/test-stream2-finish-pipe.js

@@ -1,7 +1,6 @@
 'use strict';
 require('../common');
 const stream = require('stream');
-const Buffer = require('buffer').Buffer;
 
 const r = new stream.Readable();
 r._read = function(size) {

test/parallel/test-vm-cached-data.js

@@ -3,7 +3,6 @@ require('../common');
 const assert = require('assert');
 const vm = require('vm');
 const spawnSync = require('child_process').spawnSync;
-const Buffer = require('buffer').Buffer;
 
 function getSource(tag) {
   return `(function ${tag}() { return '${tag}'; })`;