test: switch to use common.fixtures.fixturesDir

code and learn 2017 first task is to switch from common.fixturesDir to
common.fixtures.fixturesDir in test-https-strict.js

PR-URL: https://github.com/nodejs/node/pull/15814


Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>