test: upgrade from fixturesDir to fixtures.path

The common/fixtures module provides convenience methods
for working with files in the test/fixtures directory.

PR-URL: https://github.com/nodejs/node/pull/15960


Reviewed-By: Daijiro Wachi <daijiro.wachi@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>