test: use fixtures module

Replace usage of common.fixturesDir with fixtures module in
test-https-agent-disable-session-reuse.js.

PR-URL: https://github.com/nodejs/node/pull/15901


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>