test: use fixtures module

Replace common.fixturesDir with fixtures module usage in
test/parallel/test-tls-fast-writing.js.

PR-URL: https://github.com/nodejs/node/pull/15808


Reviewed-By: Ryan Graham <r.m.graham@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>