test: update test to use fixtures.readKey

Use fixtures.readKey() rather than common.fixturesDir in
test-regress-GH-1531.

PR-URL: https://github.com/nodejs/node/pull/16811


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>