benchmark: fix fs\bench-realpathSync.js

Make it call-site-cwd-independent.

PR-URL: https://github.com/nodejs/node/pull/11904
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

benchmark/fs/bench-realpathSync.js

@@ -3,6 +3,8 @@
 const common = require('../common');
 const fs = require('fs');
 const path = require('path');
+
+process.chdir(__dirname);
 const resolved_path = path.resolve(__dirname, '../../lib/');
 const relative_path = path.relative(__dirname, '../../lib/');