test: fix test_worker_terminate_finalization

The test was missing an initialization of the global `ref` variable
because there was also an unused local one, leading to failures
like the one seen in https://github.com/nodejs/node/pull/34625.

PR-URL: https://github.com/nodejs/node/pull/34726


Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ricky Zhou <0x19951125@gmail.com>