test: replace common.fixturesDir with fixture

PR-URL: https://github.com/nodejs/node/pull/15940


Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>