test: use common.fixtures module

Replace common.fixturesDir with usage of the common.fixtures module.

PR-URL: #15891
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>

test/parallel/test-fs-buffer.js

@@ -1,6 +1,7 @@
 'use strict';
 
 const common = require('../common');
+const fixtures = require('../common/fixtures');
 const assert = require('assert');
 const fs = require('fs');
 const path = require('path');
@@ -28,7 +29,7 @@ assert.throws(() => {
   fs.accessSync(true);
 }, /path must be a string or Buffer/);
 
-const dir = Buffer.from(common.fixturesDir);
+const dir = Buffer.from(fixtures.fixturesDir);
 fs.readdir(dir, 'hex', common.mustCall((err, hexList) => {
   assert.ifError(err);
   fs.readdir(dir, common.mustCall((err, stringList) => {