test: rename regression tests more expressively

- Rename test-fs-truncate-GH-6233 to test-fs-truncate-clear-file-zero
- Rename test-process-exit-GH-12322 to test-process-exit-handler

PR-URL: https://github.com/nodejs/node/pull/19668
Refs: https://github.com/nodejs/node/issues/19105


Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>