test: move buffer-as-path symlink test to its own test file

The buffer-as-path test for fs.symlinkSync() is a bit unusual and
potentially error-prone embedded in the general fs.symlink() test. Move
it to its own test file.

Refs: https://github.com/nodejs/node/pull/34540/files#r463168354

PR-URL: https://github.com/nodejs/node/pull/34569


Reviewed-By: Pranshu Srivastava <rexagod@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Gerhard Stöbich <deb2001-github@yahoo.de>