test: remove flag for test-addon-uv-handle-leak

test-addon-uv-handle-leak only requires worker_threads for the
subprocess which it explicitly calls with --experimental-worker. The
main test itself does not need it. Remove Flags: comment and move
loading of worker_threads into subprocess-only logic.

PR-URL: https://github.com/nodejs/node/pull/25327


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Daijiro Wachi <daijiro.wachi@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>