test: replace `common.fixturesDir` usage

Use `common/fixtures` module instead in test-http2-client-upload.js

PR-URL: https://github.com/nodejs/node/pull/16800


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>