test: use fixtures.readKey in https-timeout-server

Replace `common.fixturesDir` usage in test-https-timeout-server.js with
`fixtures.readKey()`.

PR-URL: https://github.com/nodejs/node/pull/15871


Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>