test: use fixtures module in test

Instead of using common.fixturesDir, uses the fixtures module in
test-http-get-pipeline-problem.

PR-URL: https://github.com/nodejs/node/pull/16117


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Daijiro Wachi <daijiro.wachi@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>